Best gear lube for 1920s vintage cars is 600W gear oil. Available from Restoration Supply Co. in Escondido, CA.
link: www.restorationstuff.com
They have an excellent online catalog. Phone 1 800 306 7008. Excellent service.
Their part No. for 600W = LUB008 (quart bottles). It is a Lubriplate product, # SPO-299
RSC also stock special lube oils for antique car steering gear boxes. I use the Lubriplate one, RSC # LUB120. It is a semi-fluid grease, NLGI No.00. The Lincoln service manual calls for gear oil, but it leaks out of the gear output shaft and makes a mess on the garage floor. The LUB120 does not leak. There is a similar Australian product called Penrite Steering Box Lube for the same purpose RSC #LUB009.