Forget about oil additives: none of them are worth anything, and some of them (especially the PTFE mixtures) do positive harm. You will not be able to "restore compression" with an additive. Use a good motor oil at the correct weight and you'll be set. Best of all, use Mobil 1 synthetic (either 5-30 or 10-30), Valvoline synthetic, Amsoil or Redline.

Good gasoline additives, used occasionally, will clean the injection system very well. Recommended by Brickboard users: Techron, Amsoil, BG-44, Valvoline Synpower, and STP in the silver bottle. The Amsoil and BG are only available from those dealers and are a little pricey. Techron may be had at Walmart and works wonders.

Don't forget to use a good gasoline as well. I usually use Amoco (midwest brand) which keeps everything clean and costs about 7 cents per gallon more. Every time my kid fills up with Kum & Go (convenience store) gas, the idle drops, the tailpipe smells of sulfur, and the car seems to clog up.