You're probably aware of the varying opinions about older engines and the use of Zinc. (or you soon will as you read up on it) Personally, I'm a definite advocate for using the Zinc additive! It's kind of hard to refute the science of it but there's always someone who swears they haven't used it and their engine is fine. (Personally, I think they either don't drive much or haven't seen the repercussions of a worn valve seat from not using it)
Anyway, all great options noted by Ted up there. I'd like to add in that Royal Purple and even Shell (Rotella) have good options available that have good quantities of zinc in them.
Don't bother with the rat-hole of how the brands differ on the amount they have - just make sure it's in there!
Also as brickwhite noted - a newer engine and some crate engines have 'roller lifters' or a 'roller cam' and they do not really need the zinc additive if you ever go down that upgrade path.
100% great info and advice so far. Current Rotella, while having more ZDDP that regular gas engine oil, still does not have enough for our old flat tappet engines. Go with a good 5/10/15W40 (and yes, Rotella T4 is a nice choice), but add 1/4 bottle of Lucas engine break in. This will give you the correct amount of ZDDP.
Of course, many of the above mentioned specialty "off road"/racing oils are very good choices that don't need the extra Lucas added. They are expensive, though.
Look at Champion brand racing oil. It is expensive but comes highly recommended by one of the nations top engine builders. If you have a Sunday driver and have already put mega bucks in your car the difference between conventional oil and something like a Champion racing oil is nothing - $25 every 5k miles?? Not really worth discussing in the grand scheme of things. I did hear from the same builder that his dyno work indicated to stay away from Driven racing oil.