My Atomic Four manual says 80,90 gear oil. Walter Gear now says SAE 30 HD motor oil...
I have been using 90wt Gear Oil but I'd like to use SAE30 HD motor oil since it would make the Vee Drive less drag.
In winter I can hardly turn the prop shaft by hand the 90 weight oil is so thick.
But- My Vee Drive is original and it weeps a little 90 weight at the seal. I bet the SAE 30 would drip.
An overhaul by Walter is about a Grand- Thats a lot just for a seal. To get to the seal a special spanner is required. My friend has one for his Ducati Motorcycles- but we tried and it was too tight. I didn't want to make a mess in Spring with a new engine so I just went with the 90 oil again.
It is down my to-do list since it works fine and isn't deteriorating- But if you can use the SAE 30 HD it will boost fuel economy.