The thread I started on coolant filtration brought this one to mind, so I thought I'd state where I stand on this stuff, and invite discussion.
Let's see, I'll start where I already started:
Coolant filtration: A totally neglected but important item. All kinds of junk gets into your coolant. Core sand, bits of hose, bits of corrosion. Best to filter it out before you have to unclog your heater core, or worse. Bypass coolant filters are available at heavy duty truck shops, and don't take up much room. They're worth every cent. Particularly if you expect your 5 year coolant to actually last 5 years you NEED to filter it!
Diesel coolant anti-cavitation additives: Worth the money if you want to avoid cavitation problems, and some diesel engine manufacturers require them to maintain your warranty.
Oil additives: The vast overwhelming majority of them are snake oil intended to separate consumers from their money. But there are a few exceptions. Marvel Mystery Oil performs mostly as advertized. I wouldn't run it all the time, but if you add some a couple of hundred miles before you change your oil it will clean your engine. From the aviation side of the house there's AvBlend. Again, it performs as advertized, but since it's PMA and FAA approved it's expensive. If you're running synthetic oil then don't bother with them.
Engine treatments: Again, most of them are snake oil, but a few are worth the money. The two that come specifically to mind are Slick 50, and Greased Lightning. Both of them are PTFE resin treatments, that basically put a thin Teflon coating on your bearing surfaces. They perform as advertized.
An aquaintance of mine, a fellow pilot, flies for the Coast Guard Auxiliary out of Clearwater Airpark, Clearwater Florida. He treated his airplane engine with Slick 50 back when Slick 50 was still offering an aviation product. About 6 months later, while he was flying a patrol about 20 miles offshore of the Big Bend area, the oil filler tube came loose from his engine, and spilled all of his oil out. He flew the plane at 55% power with no oil pressure for 45 minutes to get it to an airport. Upon inspection there was no damage to the engine, and he is still flying around behind the same engine today. I know other guys who have had similar problems, and have had engines sieze on them after less than 10 minutes with no oil pressure. I'm sold on PTFE additives.
Synthetic oil: It's worth the price. The extra lubricity of the oil really can make a difference, and its detergent qualities will keep your engine clean. Particularly if you use it with a bypass oil filter the extended drain intervals will make up for the extra price of the oil.
My own little truck is an example. I switched from Rotella-T to Rotella Synthetic. With no other change at all my fuel mileage went from 26mpg to 28mpg. Basically, this means I can now get an extra 28 miles on a 14 gallon tank of fuel, or an extra gallon's worth per tank. So by the time I've gone through a few of tanks of fuel the oil has paid for itself.
Bypass oil filters: Absolutely worth the price. A friend of mine, another fellow pilot, ran a fleet of trucks, Blocker Transfer, for 30 years. He ran Frantz bypass oil filters in all of his trucks, and very rarely had to change the oil in them. He and his wife have a matched pair of Oldsmobiles, his with 177k miles, hers with just over 150k. Both of them have had exactly 1 oil change. Both of them have Frantz bypass oil filters. They change the filters every 5000 miles, and pull an oil analysis. They run the same Delvac oil that they run in their trucks, and the oil is so clean you have to turn the dipstick to the light to see the oil level. What's really cool about the Frantz filters is that their filter elements are really cheap. They are simply rolls of toilet paper. If you pull oil analysis then you wind up changing the oil when you really need it, not when some arbitrary mileage or time figure elapses.
Fire away! :-) AP