Additives are largely BS. If there were anything you could add to your oil to make it better, Mobil, Castrol or Ashland would have already done it. Additives like Rislone for a specific purpose like flushing out engine crud, freeing up lifters, etc. may or may not work but they should not be left in the engine. Run them for a while till they accomplish their purpose then change the oil. Slick 50 et al. can destroy engines by clogging oil galleys. Chlorine based additives like Prolong reduce friction but they corrode bearings. Just run a good quality oil and change oil and filter regularly and you've got it covered.
Radiator additives? The leak stoppers, etc. tend to crud up good radiators as well as bad. Redline Water Wetter is a good thing, if you don't need glycol for low temperature freeze protection. Plain water is a better heat transfer fluid than water/glycol brine but it hasn't sufficient lubricity to keep your water pump alive. You can simply add water pump lubricant which solves that problem, but Water Wetter also increases the heat transfer capacity of the water, a major plus in a race engine.
Basically, if there's a mechanical problem you need to fix it. Additives are at best a bandaid, at worst aggravate the problem.
