Synthetic Gear Lube??

Has anyone used one of the synthetic gear lubes like MOBIL 1 75W-90W in the rear end of their older car ('39 and newer)? Also, does anyone have any experience in using this same kind of synthetic gear oil in an older straight stick transmission (on that originally speced an EP 90W oil)? Thanks, John in Wisconsin

John, I have been using 90-140 gear lube in my 40 for 30 years, and it works fine. The synthetics are even better,but you may find that your seals will allow some seepage. The synthetics will find a way out.

I have used synthetic's for years in all my vehicles old and new, from the engine oil to the gear oil with no problems. Many many years ago the synthetic lubes did seem to leak a little however they now contain an additive that prevents leakage and helps lube the seals. The old wifes tale just won't die regarding leakage.

I recently switched my recently purchased 5,748 mile Amphicar over to it and it starts easier, runs smoother and most of all operates approximatly 12 degrees cooler.

The automatic transmission fluid in my pick up that I tow my 24' enclosed car trailer seemed to be discolored and have a burnt smell even after only traveling 9,000 miles. A switch to synthetic stopped that plus the engine runs cooler and the transmission seems to shift better.

We used it in the 1963 Studebaker Avanti race car we ran at Bonneville Salt Flats in 1993. We were looking for the lowest friction possible in all areas, trans (4 speed B/W T-10) & rear axle. It worked well, car set a record over 200MPH. See the story at link below, Stude8

http://www.aoai.org/bonville.htm

With gear lube you have to consider not only weight (90, 140, etc.) but also spec (GL4, GL5). Most, if not all, synthetics are GL5. GL5 is not recommended for brass/bronze parts as it is acidic and will attack them. This info was given to me by the Valvoline tech line when I inquired about the use of synthetic gear lube in a Ford T rear end. Needless to say I went with the GL4 instead.