So I know a lot of people think additives are snake oil. And I'm inclined to agree that the most expensive ones are a complete waste of money, some of them being nothing more than colored mineral oil. Others with Teflon in them might actually clog up your motor and do more harm than good over the long-run. But I've also found a few that I really like. Here's my list, feel free to add yours.
Lucas Trans Fix: Makes the bands swell, thickens the fluid and stops leaks. If you're getting clunking sounds and slipping, a bottle of this might save you from having to rebuild the tranny, for a while anyways. The automatic in my old Bronco was hard shifting and this made the problem go away for 30,000 miles. Eliminated the slipping problem with the GM automatic in my S80 too. I've never used it in a manual/stick shift though.
Bar's Rear Main Seal Repair: Contains esters commonly used as plasticizers, the stuff you lose when seals and gaskets get stiff and brittle. Also contains a fairly strong detergent though, so I'd recommend using it when you're down a quart before an oil change, not for a whole change cycle.
STP Oil Treatment: Thickens the oil and has a bunch of zinc in it. I sometimes put one of these in when I'm little low as it restores viscosity, especially in the hot summer months.
Gunk Motor Flush: If your motor is really sludged up, this will definitely strip it clean. Follow the instructions though and leave the drain plug out for a while to remove as much residue as possible, or do the next oil change sooner than usual. You don't want to leave any of that stuff in there.
My problem with products like Seafoam is all the smoke they produce. I'm afraid all that soot will clog up the little holes in my O2 sensors and the catalytic converter, which would be an expensive mistake. I'm sure it's great at cleaning deposits out of the combustion chamber but a good gas -- Shell V-Power and Amaco/BP Invigorate are my favs -- does that too, granted much more gradually. I'm also wary of fuel injector cleaning products for the same reason. I hardly ever put anything in my gas, occasionally a little Marvel Mystery Oil if my plugs feel dirty, but that's about it.

































