Heavy oil just wears out distributor gears, put more strain on the timing chains or gear train, and in my opinion doesn't lubricate as good, its to heavy to splash around and get where it needs to be. Thicker oil can bring oil pressure up on worn out engines, but if it doesn't reach the top of the engine while cold, you wear the valve train waiting for it to warm up.
I have never ever seen a worn out distributor gear from running straight SAE 30, that is all my dad ran in the day. And again those gears have much better protection from the thicker oil, since viscosity is what gives the lubricant its load carrying capabilities. And as mentioned above it will cling better for the cold starts so no worries about reaching the top end. Top end? Just look at some very old and large marine engines the top end was lubed by way of oil cups and a man with an oil can.
Besides even a very viscous oil like SAE 60 will be almost like water when at operating temperature, well yeah a bit thicker but.
Oh I worry more about the roll pin that some like ford uses for locating the distributor gear than the gear itself.
