One of the ongoing challenges since I restored my ’36 Chevy PU in the early 1970s is finding something that matches the SAE 160 gear oil that was specified for the transmission “back in the day”. This matters because the primitive 1932-36 Chevy Master car and pickup 3-speed transmission has a small 3rd gear synchronizer that works poorly at best and becomes worse with less viscous oil.

It really bad with SAE 90 gear oil, a little better with SAE 140 and slightly better yet with so called “600W” oil that is not a gear oil at all but is a steam cylinder oil. I’ve prepared 2 different transmissions with new synchronizers shimmed to the middle of the specified 0.025” — 0.035” end play and new shift forks and they behave identically.

Pressing on I tried Mobil “600 XP460” gear oil with an ISO viscosity of 460 and that was actually worse than the “600W”. This oil is available in gallon quantities but the next step up in viscosity in this Mobil oil, XP 680, is available only in a minimum 5 gallon quantity, too pricey for a quick experiment.

Looking further I found that Lubri Plate has an ISO 680 oil packaged in quarts and Lubri Plate themselves and others sell it in 1 quart quantities. So I just got a couple of quarts of this oil to try. Its SAE viscosity is approximately 250. It doesn’t appear to be vastly more viscous than Mobil XP460 so maybe these viscosity numbers are not linear.
On a quick drive this morning of only about 5 miles the shifting did seem improved, but it was a pretty cold morning, about 40 degrees F. Only a longer drive where everything gets up to normal operating temperature will be conclusive. If anyone is interested in the results of this experiment please let me know.

Ray