Rear Axle Gear Oil - 75w-90 vs. ???

[Lynnhowlyn]

Working on the rear axle that came with my Anniversary Kit.

Build manual specifies "75W-90 non synthetic gear oil that meets GL-5 or GL-6". Having a huge difficulty in finding 75W-90 non synthetic - but "80w-90 non-synthetic" being relatively easy to find.

OK to use the "80W-90" or keep looking for the 75W?

Thanks

Lynn

[AC Bill]

Not sure why your manual specifically says not to use synthetics? Never heard of any issues from using them?

In the past I had read several threads of builders being quite happy with the Royal Purple gear oil. It is available in the 75-90, but does contain synthetics. This brand already contains friction modifiers. Red Line is another popular gear oil, that some builders use, again it is synthetic and already contains the friction modifier.

If you use a dino oil product, don't forget to add the friction modifier.

http://www.ffcars.com/forums/17-factory-five-roadsters/200758-trak-lok-differential.html

http://www.royalpurple.com/gear-oil.html
http://www.redlineoil.com/product.aspx?pid=133&pcid=4

[SirAustin]

I just went threw this same process building me ford 8.8 89 mustang rear. I talked to multiple people about this. One said that you shouldn't use synthetics if you are using a slilicon seal on your diff cover.
I called motive gear and was told to use 75-140 synthetic by one guy, and then called the next day to double check and was told 80-90. My guy that built my rear end told me to use 75-140 so thats what I went with.
I did find 75-90 non synthetic made by valvoline at napa.
Hope this helps.