This is probably a pretty stupid question but is primary oil the same as the engine oil ??

 

The big twins have three holes. One for engine. One for tranny. One for primary. You can use one oil for all three holes but most don't. Engine oil for engine. Gear oil for tranny. Several choices for primary.

 

HD's says their Formula+ oil can be used in the primary and transmission, but not the engine. However, Syn3 can be used in all 3 holes. Just depends on the oil.

Lots of folks run 20W-50 synthetic engine oil in all 3 holes with no issues, but I don't.