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I've used Sea Foam on several different cars, all of them over 140,000 miles. One of these had been badly neglected, so that it took a coat hanger to scrape the sludge out of the oil pan. I ran a change of cheap oil for about 500 miles, drained it, put in some more cheap oil with a whole can (NOT the recommended procedure!) of Sea Foam, ran it gently for about 10 miles and drained it into a clean pan.
I figured if it looked good, I'd pour it back in. It looked like ink. Completely black. So I replaced the filter and put more cheap oil in it with Sea Foam. This time, after about 50 miles I drained the oil and it looked normal, so I guess it had done whatever cleaning it was going to.
I don't recommend this procedure unless you have a motor that's badly gunked up, and even then, you have to be ready to give up and tear it down. I figured the Sea Foam as a "kill-or-cure" solution, and it cured it. If you use it as recommended, it should be safe enough, though.
I used some in a motor that was in better shape, and it didn't seem to make any difference.
If you want to gently clean out a motor, and keep it clean, use an oil mixed for heavy-duty diesel engines. Shell Rotella, Mobil Delvac, Chevron Delo, Valvoline Blue, there are others. They have strong additive packages, and are made to control sludging in engines that put more soot and crud into the oil.
The HDEO (Heavy Duty Engine oil) will clean the engine out slowly, so you don't run as bad a risk of dislodging something and having it block an oil passage. Plus, they tend not to be as expensive (at least in a synthetic) and you can safely run long OCI with them (use oil analysis to verify). They usually have a cheaper 15w-40 dino that works ok in hot weather if it's compatible with your engine, and a 5w-40 synthetic that works year-round in most motors.
I've used 0w-30 and 5w-40 in the same motor and not noticed any mileage difference, but there were other variables involved. I will say the engine keeps the oil pretty hot, so that might reduce the impact of the 30w vs. the 40w. The 5w-40 does make the engine idle quieter. I think it's valvetrain noise that's different.
I ran the 5w-40 Rotella T6 for 5500 miles, and it still had a good TBN (total base number-the oil's ability to neutralize acidic contamination), flash point and viscosity. The oil analysis guys recommended trying a 7500 mi OCI. The 0w-30 Mobil 1 at 5000 was used up (TBN, viscosity and flashpoint all at the limit), and they recommended not exceeding 5000 miles.
Since the 5w-40 is longer-lasting and cheaper, I'm going to stick with it. (8 quarts of synthetic at oil change time is no joke!) The 0w-30 is a gas-engine synthetic oil, not an HDEO.
I think Rotella (and maybe others) is also available in lighter weights that still have the strong additive package.
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