[Ross F-1]

I did a bunch of web searching on this about 10 years ago because I had a vehicle that the PO had used "corrosive" 90wt in. The sulfur compounds only become a problem at temperatures well above something like 200 deg. F. Back then, the Dodge Cummins crowd had bunch of failures from towing heavy 5th wheels thru the desert in the summer, their trans temps were WAY up there. I kind of doubt any flathead vehicles can generate that high of a trans temperature. But it can't hurt to avoid oils that could hurt.