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Most newer oil "low emissions" has less zinc and sulfur these days. I remember Norm suggesting before to add Marvel Mystery oil after every oil change to ensure enough cam lubrication. The newer oils without Marvel can cause flat tappet cams to wear prematurely.
Also it depends upon your climate (temp).
I use 20-50 Valvoline Racing or Castrol, but your climate may dictate a lower or higher viscosity as climate dictates.
More than you may care to know about oil and lubrication...
Personally, I think the zinc thing is overblown except during break-in. Seems that the biggest proponents of ZDDP also sell it.
That said, I will break in my new Poly on dino with additives then switch to synthetics. May add a little STP red for zinc.
DO NOT use synthetics in a newly rebuilt "old school" motor until you have at least 7500 miles on it.
The rings won't seat properly
I agree with all that's posted if you assemble it all together: viscosity per climate, consider additives, in England Castrol is your most likely choice for top quality, but synthetics are fine too. The caution on holding off using synthetic until after complete break-in is dead right, or your engine will be dead-dead not much later.
I like Lucas oil additive. Its like the old STP oil treatment. I also use Rotella T 10W30 by shell in the cuda. Its a heavier duty oil.
Personally I also take fish oil, 2 pills a day. Fish oil is an EFA (essential fatty acid) that increases your HDLs, the good lippo protiens that carry cholesterol out of your blood back into the liver. Its also a joint anti-inflammatory. Maybe putting a few pills in the old 318 would do it some good, eh??