regular to synthetic oil..

The new ring and pinion I installed is calling for 75-140 synthetic gl-6 or higher rated , instead the 85-90 regular gear oil for the rear end.The manual trans uses the same fluid as the rear ....would it be ok to switch the manual trans to the synthetic 75-140 also?:confused:
or maybe use a lighter synthetic closer to the original weight for trans ,if the weights even cross over between synth and regular oil?

If the Tranny calls for 80w-90 gear oil I would try to keep the same grade. We have switched many trucks over to synthetic gear oil, we tend to use 75w-90 or atleast what the manufacturer recomends. We haven't had any problems, other than the small leaks tend to get bigger lol

Wanna try an idea? Find your BG rep. Get the synthetic. You will not be dissappointed no matter what your driving conditions.

What brand gears are they and what vehicle is this for?
Here is some interesting reading on GL6:
"In researching API service classifications for gear lubricants the fact that API-GL6 gear oil, while admittedly superior in extreme pressure situations to those of lower GL-numerical order (i.e., GL5, GL4, & GL1), is (apparently) now considered to be an obselescent service classification (along with GL2 & GL3).

Looking up why, it appears the loss of API GL6 tested & designated gear oils seems to have both practical & theoretical implications.
That (apparent) lack of API GL6 oil on the market is explained in the aforementioned API PUBLICATION 1560 SEVENTH EDITION, JULY 1995 publication titled "Lubricant Service Designations for Automotive Manual Transmissions, Manual Transaxles, and Axles"
The designation API GL-6 denotes lubricants intended for gears designed with a very high pinion offset. Such designs typically require protection from gear scoring in excess of that provided by API GL-5 gear oils. A shift to more modest pinion offsets and the obsolescence of original API GL-6 test equipment and procedures have greatly reduced the commercial use of API GL-6 gear lubricants."

I question why the gear mfg is suggesting GL6. Pretty much any Full synthetic GL5 is going to perform fine and gear wear will pretty much be nill after break-in. I would suggest dino based gear lube of suggested viscosity initially to allow the gears to break in properly before switching to a full synthetic. FWIW, Amsoil Severe Gear in 75W-140 is a pretty dang good gear lube and it is a FULL synthetic based lube.

Thx fellas,I went with 85-90 oil in the trans....and used the 75-140 lucas in the rear,both non synthetic.