GL-4 Spec Oils

[bunnspecial]

In the MG circles, there exists a lot of debate about how appropriate GL-5 spec oils are in the differential. The fear is that the higher sulfur content of GL-5 oils can cause issues with the copper alloy thrust washers in the differential.

Does this argument have any merit? Of course, if one goes shopping at the usual sources, most gear oil one finds on the shelf is some flavor of xxW-80 or xxW-90 GL-5. The often recommended GL-4 EP90 seems rather thin on the ground, or at least in the quart quantities more appropriate even if one changes on a fairly aggressive interval(6000 miles per the manual). The only GL-4 oil I've ever personally had in my hands was Redline MT90, and I'm not sure if that's even an appropriate oil for the differential. I know some big MG shops that keep EP90 GL4 on hand, but they usually buy it by the 5 gallon bucket and can use that amount in a reasonable amount of time.

Does anyone here have any thoughts as to whether or not the GL-5 concern is overblown? My inclination would be to just toss a good synthetic 80W-90 or so in it and call it a day until I get motivated enough again in a few years to change it again.

 

[MolaKule]

What is the design of that differential. Hypoid or worm gear?

Does it have any bronze or brass parts in it?

Most modern Hypoid differentials have nothing but steel parts in them,

 

[bunnspecial]

It looks hypoid to me(diagram courtesy of Moss Motors)

And the thrust washer(67 above) certainly looks brass to me

 

[Lubener]

You want nothing but a GL-5 gear oil for your hypoid gears in your differential.

 

[Jetronic]

Use a GL-4/5 rated product, or even better a total driveline product. The latter will ensure you have GL-5 protection for the steel parts but MTF compatibility means kind to brass.

 

[Rand]

most major brand name gl-5 is ok there were some corrosion tests done a few years back.

IIRC lucas and Royal purple were on the bad end.

 

[MolaKule]

It looks hypoid to me(diagram courtesy of Moss Motors)

View attachment 25040

And the thrust washer(67 above) certainly looks brass to me

View attachment 25042

Looks like a copper plated steel washer. Copper does not have enough strength to flex or to provide 'thrust" capabilities.

 

[endeavor to persevere]

Red Line 75W90 GL5 does not harm yellow metals according to Red Line. But I don't think Mobil 1 75W90 GL5 does either.

 

[MolaKule]

Red Line 75W90 GL5 does not harm yellow metals according to Red Line. But I don't think Mobil 1 75W90 GL5 does either.

All gear oil formulations have a chemistry called "metal inhibitors" or buffer to keep any sulfur components from causing corrosion.

Chemical, electrochemical, or catalytic reaction between a material, usually a metal, and its environment, produces a deterioration of the material and its properties. We therefor use an additive agent in lubricants called a Metal Inhibitor to reduce or eliminate any any of these effects.

https://www.bobistheoilguy.com/forums/threads/fill-in-the-blank.328782/

A Review of Engine Oil Additives

Engine Oil Additives from STLE

www.bobistheoilguy.com

 

[Jetronic]

If the phosphate glass film is stronger than the copper it's forming on, every time the glass layer gets stripped it will take some copper with it. Doesn't need to be corrosion wear.

Widman has some opinions on this exact subject https://www.widman.biz/Corvair/English/Links/Transaxle.html

 

[GJM120]

Because of the way the additive pack adheres to the metals, if there are softer metals in the gears you want GL4 according to this paper: https://www.widman.biz/uploads/Transaxle_oil.pdf

The bottom of p1 is where the basic explanation is given. I've read the same conclusion elsewhere.

FWIW I'm using a GL4 in my 1966 Triumph Bonneville gearbox.