best oil to use in the diff?

[Dan76spit]

hello all,
i am about to replace the diff in my 1976 spitfire with a new-to-me used unit. i drained all the old fluid out and am going to replace it before i put the diff in. what is the best fluid to use in them? i know the manual says gl4 90 weight, which i can find locally at napa, just wondering if this is the best stuff or is there something better? I know its prob a matter of opinion but figured i'd see what everybody else thinks as i am no expert. thanks...

 

[hermanmaire]

I would stick with a name brand 80w90 oil. I would not bother with any synthetic oils.

 

[TR3driver]

Personally, I'm a big fan of Valvoline full synthetic GL-5 for diffs. Used it for 20 years in my TR3A, and it has worked great. It obviously can't cure any mechanical problems you already have, but I credit it for making my diff live as long as it did (over 150,000 miles and it was well worn when I got it).

But that's just my $.02. YMMV and all that.

 

[Andrew Mace]

See also this recent thread on virtually the same topic! Nothing at all against synthetics, so long as they DON'T contain the ingredient that tends to dissolve the yellow metal shims, and again make sure you're using an extreme pressure (EP) lubricant regardless.

 

[tdskip]

Guys - didn't we decide that GL4 was the ticket since there was some brass in the diff after all?

 

[TR3driver]

Guys - didn't we decide that GL4 was the ticket since there was some brass in the diff after all?

Depends entirely on who you mean by "we". I spent quite a bit of time researching the topic some years ago; and while the business about GL5 attacking "yellow metal" is not entirely a myth, it is not nearly the whole truth either.

In a nutshell, there is a particular additive that can be used to improve the "extreme pressure" performance of the gear oil (rather like ZDDP works in motor oil). Neither the GL4 spec nor the GL5 spec say anything at all about whether to actually use this particular additive, nor how much is allowed (although they do both have limits on how corrosive the oil can be). Some gear oils (both GL4 and GL5) use this particular additive, some do not. The kicker (and origin of the myth) is that <span style="text-decoration: underline"><span style="font-weight: bold">some </span></span>GL5 oils use rather a lot of this additive, which can break down under high temperature and pressure to form sulfuric acid, which is what attacks the yellow metal.

So the best course, IMO, if you want extra-long life from your diff, is to look for an oil that does not use <span style="text-decoration: underline">any</span> of that particular additive. Most 'synthetic' gear oils do not, and in particular the Valvoline brand full synthetic that I use does not. One way to tell is to look for the "MT1" rating (in addition to GL5). MT1 has nothing to do with differential service (it is for heavy duty truck transmissions that don't even have synchronizers), but it does have a long term stability test that the "active sulfur" additive cannot meet (because it breaks down).

 

[Mark Jones]

I have been using fully synthetic GL5 gear oil in my Spitfire's differential and transmission for 8 years now with no ill effects. I do look for one that stated that is it compatible with yellow metals (just to feel more safe).

I have looked into the API service designation for GL5 and it does state "...protection against oxidation, thermal degradation, rust, copper corrosion..."

 

[Dudly]

Been using the GL-4 from NAPA since I asked this very question a couple years ago.