When rebuilding my 261, I took the advice of many others and used the GM EOS supplement as an assembly lube. Then, when I was ready to start it, I dumped the rest of the EOS in the crankcase and included a quart of Lucas oil. Then filled the rest of the crankcase with Valvoline straight 30 wt oil which supposedly had more zinc than other oils. Now that the break-in is done, I think that I'll also add some Lucas and possibly the EOS stuff to the Castrol 20-50 GTX I favor. Hopefully, with the few miles I put on the truck each year, it'll outlast me.
