Do engine treatments like Slick 50 really work?

I would say no, but advise you to search ftc.gov for “Slick 50” and read supporting documents there.

About 20 years ago, I decided Slick 50 sounded like a good idea, so I put some into a brand new Jetta GLI, with no ill-effects. I remember it was expensive, and at the time, I think it had teflon in it. Teflon in a crankcase is a bad idea; it can clog journals quickly, and do the opposite of reducing wear. Luckily, that problem never happened for me.

Just read your owner’s manual and follow the recommendations there. That’s your best course of action. If it says “5W-20 synthetic, service grade SM”, use that. The manual for my 96 Maxima specifically recommends 5W-30, non-synthetic oil, so that’s what I use.

Some manuals even go so far as to specify using no additives.

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