As I recall, 600W was sperm whale oil which became illegal in mid 1970's..

In my past life, we used Steam Turbine oil in gearboxes. For anything with brass
or bronze parts in it, lubricants of the GL-1 type are often used. Sometimes sold
as worm gear lubricant. This becomes a factor if the loads cause heat in the brass.
Just idling along might not matter. There might be a black market for your 600 drain oil.
For a century it was used for everything from sewing machines to automatic transmissions.
Now Jojoba oil replaces it, but costs a lot, like $25 per quart..

Where Are You From? MN

Do You own a car built by Durant? yes