The zinc compound abbreiviated ZDDP, is an extreme pressure additive that helps keep flat tappet camshafts from wearing out.
About a year ago, all gasoline oils went from the old SL spec to the new "emissions" SM forumulation. This is the grade that is in the little circle on the back of the bottle.
Anyway, this was done because it was found that the anti-wear additives -could- cause problems with catalytic converters if the car burned a lot of oil. This mostly concerns cars with over 100k miles.
The new oils have a lower concentration of zinc and phosphorus compounds. Bad news on the Valvoline Racing Oil... It held onto the older SL rating for a while, but the latest stuff I saw at the store had the emissions SM rating. This was both the 20w50 and the straight 50.
Diesel oil has traditionally had more anti-wear compounds than gas oil. However, with the current low emission diesel engines that also use a form of a catalytic converter, their oil has also changed. The old pre-emissions oil had a CI-4 rating. The new stuff has a CJ-4 rating. The new rating wasn't done THAT long ago, and some CI-4 oil is still on the shelf. THIS is the stuff to use in flat tappet engines! CHECK THE RATING!
As for additives. EOS, and every other "street" oil supplement containing large amounts of zinc and phosphorus compound have been DISCONTINUED. General Motors EOS and the 4-cyl STP in the red bottle are now out of production. If you want these additives, you've got to turn to the race shops. Comp Cams sells a very popular additive. It's part number 159. Crane also offers their additive.
My advice is to stock up on CI-4 diesel oil while some of it is still out there. Some stores still have CI-4 oils. Shell Rotella 15w40 went to CJ-4, but the straight 30w is still CI-4. About a month ago, I saw Rotella CI-4 15w40 in 2.5 gallon jugs at Tractor Supply. If you have a flat tappet engine, use diesel oil, and try to use the CI-4 stuff.
As for affordable synthetic oil... All Mobil 1 has the current "emission friendly" SM rating EXCEPT their "High Mileage" oil. This still has the older SL rating. It is available in 10w30 and 10w40, and I just purchased some at Wal-Mart for $19.95 for the 5-quart jug.
Of course Royal Purple is a "take no prisoners" oil. If you truly want the best, and don't mind spending the money, this is the stuff for you.
Below is a link that has a chart of different oils and their additive amounts.
Oils: What motor oil should I use? Which oil is best for my Porsche or aircooled engine?
Here's an Austrailain motor oil test. Just keep in mind, this is just a test for extreme pressure protection.