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Gear oil- the verdict

Discussion in 'Suzuki Carry' started by spaner, Aug 9, 2010.

  1. spaner

    spaner Well-Known Member


    Everything is pieced together from the manuals, Internet , MiniTruckTalk and Wikipedia.

    Put the proper stuff in and not only will you not have future problems, but general operation will improve.

    Excerpt, I asked a well versed mec. friend of mine(owns and operates his own shop) today for his views on manufacturers API GL rating recommendation VS vendor availability. He said "what's a GL rating" I shi* you not...what about additives I asked, corrosive to yellow metals prevalent in, especially late model, Japanese vehicles? "been using the same stuff for years" (GEAR OIL) He didn't know what type it was he used, only that the vendor sends it over when he needs it. "But if you want the good stuff, use LUCAS"

    Just a little run on there to reinforce the fact that your local mec. does not always know everything and the more people I ask about this subject, the more shocked I am that I know more than they do, which should not be the case.

    Research and share, that's what this site is all about, and it's not always about, or for, the trucks.

    Gear oil is a motor oil made specifically for transmissions, transfer cases, and differentials in automobiles, trucks, and other machinery. It is of a higher viscosity to better protect the gears and usually is associated with a strong sulfur smell. The high viscosity ensures transfer of lubricant throughout the gear train. This is necessary since the devices needing this heavy oil do not have pumps for transferring the oil with only a portion of the lowermost gears bathed in an oil sump. This heavy oil can create viscous drag leading to inefficiencies in vehicle operation. Some modern automatic transaxles (integrated transmission and differential) do not use a heavy oil at all but lubricate with the lower viscosity hydraulic fluid, which is available at pressure within the automatic transmission.

    Most lubricants for manual gearboxes and differentials are HYPOID GEAR OILS. These contain extreme pressure (EP) additives and antiwear additives to cope with the sliding action of hypoid bevel gears.

    API ratings
    Gear oils are classified by the American Petroleum Institute using GL ratings. For example, most modern gearboxes require a GL4 oil, and separate differentials (where fitted) require a GL5 oil. It is IMPORTANT that purchasers check the oil against the vehicle manufacturer's specification to ensure it does not contain any AGGRESSIVE CHEMICALS that may ATTACK gear components, such as the PHOSPHOR BRONZE used in many Japanese boxes. (use the recommended API specification)

    API viscosity ratings for gear oils are not directly comparable with those for motor oil, and they are thinner than the figures suggest. For example, many modern gearboxes use a 75W90 gear oil, which is actually of equivalent viscosity to a 10W40 motor oil. Multigrade gear oils are becoming more common; while gear oil does not reach the temperatures of motor oil, it does warm up appreciably as the car is driven, due mostly to shear friction (with a small amount of heat conduction through the bellhousing from the engine block).

    Fully synthetic gear oils are also used in many vehicles, and have a GREATER resistance to shear breakdown than mineral oils. They can IMPROVE the shifting performance of "difficult" gearboxes, where the excessive slipperiness of some mineral oils can impede synchromesh action.

    Relavant API classifications

    API GL-4, oils for various conditions - light to heavy. They contain up to 4.0% effective antiscuffing additives. Designed for bevel and hypoid gears which have SMALL displacement of axes, the gearboxes of trucks, and axle units. Recommended for non-synchronized gearboxes of US trucks, tractors and buses and for main and other gears of all vehicles. These oils are basic for synchronized gearboxes, especially in Europe.

    API GL-5, oils for severe conditions. They contain up to 6.5% effective antiscuffing additives. The general application of oils in this class are for hypoid gears having SIGNIFICANT displacement of axes. They are recommended as universal oils to all other units of mechanical transmission (except gearboxes). Oils in this class, which have special approval of vehicle manufacturers, can be used in synchronized manual gearboxes only. API GL-5 oils can be used in limited slip differentials if they correspond to the requirements of specification MIL-L-2105D or ZF TE-ML-05. In this case the designation of class will be another, for example API GL-5+ or API GL-5 LS.

    ---------------------------------------------------------Retrieved from "http://en.wikipedia.org/wiki/Gear_oil"

    My Suzuki DD51t specifically requires:

    4WD gear box and transfer case combination (fluid is shared)---2WD same type and viscosity
    3.5-3.7 L API GL-4 75w-80(or 90)
    Fill and drain plugs located; rear facing; on the back side of the front out-put drive shaft.
    (Mine had 1L in it and it looked like mud)

    Rear Differential (locking-not limited slip)---2WD same type and viscosity
    1.3L hypoid API GL-5 75w-80(or 90)
    Plugs are visible.
    (Mine was full, but black as tar)

    Front Differential
    1.7L hypoid API GL-5 75w-80(or 90)
    Plugs are visible.
    (Mine had 0.5L in it, and looked like grey paint)


    You can "put the same stuff in the diffs and the gear box". Same viscosity but not API GL rating........
    And yes it will make a difference in driveability as well as longevity...

  2. olddatsunfan

    olddatsunfan Member

    Nice post Spaner, so which brands of GL4 synthetic lubricant are "yellow metal" friendly ?
  3. Little Foot

    Little Foot Member

    Does Amzoil fall in this category?
  4. anthill

    anthill Member

    AMSOil ran a fairly exhaustive-looking study of gear lubes. You can read the PDF here. The copper corrosion test results are on page 17. Summary: most modern GL-5 oils are buffered enough that they don't eat yellow metal. Pick a gear oil that's passed the MT-1 manual transmission test and you'll be fine. Don't use Lucas, Royal Purple, or Mopar.
    Last edited: Aug 11, 2010
  5. Little Foot

    Little Foot Member

    This was very educational, thanks very much and it looks like I made the right choice but I paid almost 15$ a litre for the Amsoil stuff. Worth saving that document to my Adobe files.
  6. Ironraven

    Ironraven Active Member

    Hmmm... good info I'll have to check the fluid I just swapped into my trans and see if I'm safe.
  7. spaner

    spaner Well-Known Member


    Nice contribution anthill, Ironicaly, the "best" recomended by my mec. friend, faired the worst...Lucas..

    I have not come across, nor did I read about, any API GL-4 that did not meet the MT-1 Mil specification...

    Has anyone else come across one?

    Ho, by the way, two things I have noticed after 48hr with new fluid in trans.

    1) Ever since I had the truck; after it had sat for more than one hour, It would NOT roll backwards in neutral with the clutch out and in reverce, it would load down the engine (like the park brake was on) to the point that I would push the clutch back in. It would roll back with the clutch in. I always had to pull farward first, just a bit.
    That problem is GONE.

    2) I have a 5 speed and have always shifted from first to second in a reverce C pattern...now I can go strait down from first to second and shifting all around is like night and day.

    Forgot to mention here...

    I used Quaker State 100% Synthetic GL-4 75w-80
    and Quaker State 100% Synthetic GL-5 Multi-Grade


    Altenate location for the PDF gear oil study:

    http://www.synthetic-oil-tech.com/Gear Lube White Paper.pdf


    Last edited: Oct 26, 2011
  8. Little Foot

    Little Foot Member


    Front differential took 625ml or 21 ounces. 10$ worth. 100mm hex nuts both drain and filler plugs
    Rear took 775ml or 26 ounces. 12.50$ . 19mm bolt for drain and 3/8" square drive for filler plug.
    So service for both diff's 22.50$ for AmsOil 75W-90

    Both diff's were full and oil was clear. Must have been changed before I purchased it a year ago. But better to be safe than sorry.

    Now the transmission I will do soon and let you guys know.
    Last edited: Aug 11, 2010
  9. Ironraven

    Ironraven Active Member

    My diffs were full of pitch black crap! There was probably about 1/2 oz of metal filings stuck to the magnetic drain plug on my trans drain too :/ someone must have learned to drive a manual trans on my truck lol
  10. fupabox

    fupabox Well-Known Member

    Good reading guys...I was taught that like regular motor oil the desisnation API GL3 was a replacement for GL2..and GL4 a replacement for GL3..and so on..so for years I have been mistaken thinking that a higher GL rating was always superior to the lower...I did know that Lucas was shite tho..saw some tests on it years ago...the little gear thing on parts desks sells a lot of inferior additives:p
  11. spaner

    spaner Well-Known Member



    Started searching with this google document search function:sly:

    Couldn't have explaned it better with a PhD and a case of beer.

    GL-4 Protects steel less and attacks yellow metals less
    GL-5 Protects steel more and attacks yellow metals more

    Reads simple...but it really isn't....GL-5 additives to counter act other additives, just stick with the GL-4, which has less additives...but then, you might not be able to find any, anymore..:confused:


  12. Little Foot

    Little Foot Member

    Drained my transmission today. A little more than ½ a liter came out nice and clean though. It took a little over 2½ liters. 2¾ US quarts. No filings on magnetic drain plug. Took it out for a drive. Quieter and shifts better from third to fourth (less clunky). Just happen to have 2 liters of Amsoil GL-4 hanging around. Topped off with the leftover Amsoil GL-5 from my differentials. So I'm running GL-4.2 :sly:
  13. o8k

    o8k Member

    This might be a bit random but. Im a mobile-1 guy. Tried Amsoil in several diesel engines I owned (was never impressed). I tried to be, but just couldnt get there. Great article. I always love reading about oil. For somthing that serves such a simple purpose, it is always a highly debated topic and a very mis-understood material.
  14. Little Foot

    Little Foot Member

    Well it's up there with Amsoil and cheaper

    Attached Files:

  15. anthill

    anthill Member

    This post is worth the cross-link. Good link to a discussion of GL4/5 oils.

    The argument there which sounds plausible is that the wear on copper surfaces is not a matter of chemical corrosion, but of scuffing and abrasion.

    The GL5 compounds cling to the brass surfaces tight enough that "the sacrificial coating is stronger than the base metal, and instead of just peeling off, it takes with it a few microns of brass that it is bound to".

    So that means that the AMSOil tests that I linked above are insufficient to tell if a GL5 oil will be safe for your old gearbox. Even if they don't corrode a brass surface, they might still abrade it.
  16. o8k

    o8k Member

    all that being said, i have GL5 in my tranny at the moment. =\ dogh! I will have to make a note to switch it out next time i have money for such things.
  17. Tony Evers

    Tony Evers Active Member

    Thanks Spaner you are a wealth of information
  18. Anne Sweeney

    Anne Sweeney Member

    How about this? This is locally available in the Philippines and the cheapest ones around. There arent many options unless we dig into hardware stores at ridiculously high price synthetics. Most dont care less about either.[​IMG]
    Last edited: Aug 5, 2015
  19. spaner

    spaner Well-Known Member

    API "GL-4",

    It's not the best stuff out there, but it will do you just fine for the gear box/trans,

    If it reads " API GL-4"; then it IS a GL-4 and will meet, or exceed the gear-box requirement and "yellow-metal" wear restrictions for HP additives...(AKA-None)


  20. TheChurchMouse

    TheChurchMouse New Member

    This information is great! Fairly new owner of a 97' Carry and loving it! Wanting to change out the transmission and diff oil, I have the manual and want to put the right stuff in but am having trouble sourcing it.

    So my question is, for my diff can I use a 75w-90 instead of reccomended 75w-80 as long as I keep it GL-5?

    Also for the transmission It calls for GL-4 and I think that should be a mt-90 I think?

    Any confirmation or suggestions would be greatly appreciated, I want to get this done asap as I believe it will greatly help with the grinding from 1st to 2nd.

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