Give me a chance to review everything you wrote...Good post.
But, be advised 10-30 is often made using the cheap,low quality base... The window of what is considered 10-30 is very wide.
I would say, as a rule of thumb, You could use a 15-40 on flat cam engines, like rovers, because in order to meet a 15-40 rating the shear ratings will be in spec.
Let me review everything else you mentioned. I have a full day today here at work.
