Originally Posted By: Beem
Originally Posted By: 440Magnum
Redline makes a GL-4 product line in various weights- MTL, MT-85, and MT-90. I would have ZERO qualms using any of those fluids in a 60s vintage transmission (let alone 80s) . Even though they are synthetic they're pretty much made for the purpose.
http://www.redlineoil.com/Products.aspx?pcid=7
If you want non-synthetic, your best (pretty much only) option is an ordinary engine oil. Use the viscosity table on the main BITOG page to determine what weight to use- SAE 40 engine oil is roughly equivalent to SAE 90 gear oil.
http://www.bobistheoilguy.com/viscosity-charts/
Despite some opinions, every vintage vehicle I've put synthetic oils in develop leaks very soon after use. Some people claim to get away with it, I never have. Can't fool me four times.
Maybe motor oil is an option as it is also noted as an option in the manual but that opens a whole new can of worms. Modern oils do not have zinc and phosphorus that the classic oils did. So a motor oil would probably need to be an old spec oil to be safe.
Thanks for the chart link. Looks like 30 wt is in the ballpark. I'm thinking before an SM rating might hit the mark.
I wouldn't worry about ZDDP content for a number of reasons- a transmission doesn't have the extreme sliding contact pressure points that a flat-tappet cam engine does. Besides, the modern (SN) levels of ~800 PPM for xw30s and 1200 PPM for xw40s are just fine. 800 PPM may be a problem for breaking in a flat cam, but its enough for sustaining one after breakin. People back in the "old" days weren't that aware of ZDDP, and it was rarely more than 1400 PPM in older oils anyway- it only became an internet phenomenon when engine builders tried to break in new cams, or run very very aggressive cams with monster valve springs with modern 800 PPM oil and ran into trouble.
As for the manual specifying "non-synthetic," I'd ignore advice given 40 years ago based on 40 year old oil technology COMPLETELY. Synthetics and conventionals behave pretty much exactly the same with respect to seal materials these days.