Reply To: Proper lubricants for 1702 Coupe

[George Teebay]

Hi Steve,

For my money, any transmission with synchromesh and especially overdrive or free wheeling units should use GL-1 (straight mineral oil, no additives). I ordered a 5-gallon pail of 90-wt from my local NAPA store and they had it for me the following day. I’m not sure whether they sell it in smaller containers, but one gallon would do you. That’s suitable for the steering box as well. The differential is hypoid and thus requires an Extreme Pressure (EP) gear oil, GL-4. Do NOT use GL-5 (EP with additional additives for limited slip differentials) or GL-6 (synthetic), as those two have additives which may attack the yellow metal (brass and bronze).

After consulting with a Sta-Lube PhD chemist, in 1996 I filled my 1936 diff with GL-6 synthetic for the purpose of cooling it better on a crosscountry trip. What a mistake! In Wyoming I crawled under the car to fill a leaky rear shock and almost burned myself on the diff cover. At the next town I bought GL-4 and a drain pan, and found the dreaded golden speckles (from deteriorating yellow metal) in the drained synthetic. Apparently I caught it soon enough, because 20K miles later there are still no apparent ill effects.

Among my non-Pierce cars, I have two Jeepsters, whose owners manuals explicitly state to use hypoid lube only in the diff, and not in the trans and overdrive–which are also Borg-Warner.

Use 600W in non-hypoid diffs such as series 80/81 (one easy way to tell is if the pinion enters the center of the pumpkin rather than near the bottom), and in non-synchro (i.e., crash-box) transmissions. If you buy 600W (not for this car!), you can get it from a Model A Ford parts source but be sure to get the dark, smelly, super-viscous stuff rather than the honey-colored, less-viscous gear oil which I suspect is re-packaged SAE 160.

In my experience, true 600W (AKA steam cylinder oil) gets thicker with time and almost tar-like after 30 years or so. I use Texaco Thuban SAE 250 which is available from big rig repair shops in 5-gallon pails.

Hope this helps!