Originally Posted by Quen
From what I can gather one main concern with the GT-40 regarding oil is zinc content. Our engines are flat tappet and need a certain amount of zinc for protection. I use Rotella 15W-40. Wikipedia lists it as having 1200 ppm.
This is incorrect. Rotella used to be high in zinc and sulphur, two critical components for flat tappet cams. The zinc cushions and protects from metal on metal content, the sulphur enables the zinc to do its job and carries it. Rotella ever since a couple years ago when they changed to the new design on the bottle is low zinc and sulphur.
I used to break in new engines with Rotella, no cam additive. Never had a problem over tens of engines built. Remember breaking in a 302 I picked up some Rotella and thought it was odd that the bottle looked different, but checked and it was the same stuff according to the bottle. Well, on breakin I wiped the cam. That's the first and only cam I've ever wiped. Moral of the story, if you want high zinc/phos avoid Rotella, it isn't the good oil it used to be.
I use Valvoline Racing VR1 for engine breakin. IIRC it's 1300/1200 phos/zinc. Should be fine on it's own but I use cam breaking additive with it. Mobil1 is another good one on Zinc/Phos but only in a couple grades of their oil. It's not well suited to cam or ring breakin though. Amsoil is another goodie, but again only in some grades. I have charts for Amsoil, Mobil1 and Valvoline I'll try to dig up.
Bottom line is in your boat it doesn't matter what oil you use as it's already broken in. Cams don't just go flat, they fail at breakin or fail due to a stuck lifter not rotating, excess valve lash, and various other wear or improper maintenance issue. Personally after breakin with VR1 I use either VR1 or regular Valvoline (depending on engine HP and use) and then switch to Mobil1 after I'm certain the rings are seated and engine is fully broken in.
Bottom line, that engine is well broken in by now, use whatever you want. Personally I would run Valvoline 10w40 with a Purolator PL30001 filter and change it every 50 hours or sooner if it gets dirty.