I pm'd Bob about it. The reason I was wanting to increase the clearance by .001 on the Ranchero engine is because of "sustained hi speed operation", as in down the salt at B-Ville......which would to me justify a looser clearance and higher viscosity. I've used Mobil 1 15W50 in lots of my engines, never had any problems with it..... but I do believe I will go to a lower viscosity on my street stuff...