the lifters do vary from year to year. For example the top of my 1990 model OEM and aftermarket ITM lifters there is only one oil galley. Also the top of the lifter is smooth and doesn't have the slot machined in them. Both the OEM and ITM lifter oil galleys have the same diameter and in general the same dimensions. I want to do a technical write up on the comparison however I am in the same boat as you and doing my second rebuild. I now have the same question: is the two oil galleys better than single galley? Maybe someone more knowledgeable can shed some light for us. Cheers!