Hello, Dallas69, you made me think of something when you said, "...my Dad never used multi wt oil so I used 40 wt.....". That was in the late 60's, you say.
When were multi-weight oils introduced? I'm thinking it wasn't too much before the late 60's.
I remember the SE oils (10-30, 10-40) being featured in Popular Science magazine in the late 60's.
That magazine made me an "expert" on many things.
What about before that? Your Dad was just being old fashioned in his time. Kira