Well, for me, I rather err on the side of safety. When finishing the 261 rebuild, I used GM EOS for all internal lube issues. Then when breaking in the cam, I filled the crankcase with the rest of the EOS, Valvoline 30 wt (which contains more zinc according to their website) and Lucasoil. Think I'll use up the rest of the Valvoline on the next oil change and possibly switch to the Syntec oil. Looks interesting. Have always used Castrol GTX 20/50 in all my engines of later vintage and have never had a failure of any sort.
