I checked the compression on my 91 1.6 with 144k miles, and am not sure what to make of the numbers.
The dry compression is (front to back) 150, 155, 160, 180. To check it wet, I added just a little oil, and there was no change. After adding a little more, it was ~200 on the first cyl, but I think I may have overfilled. It's also an approximation because I used one of those gauges that you have to press down into the spark plug hole instead of the screw-on type, and apparantly I wasn't holding it down tight enough
Since that's not particularly helpful, I'll try to re-do it using an equal amount (1 teaspoon?) of oil in each.
I also have an explanation for the higher compression on cyl 4. The #4 spark plug looks like it had never been changed, as it was rounded at the top and nearly impossible to remove. I'm pretty sure it was the orignal plug from the factory... So that cylinder wasn't really working all that well until I got it removed around 2k miles ago.
What does this mean? It seems to run fine; I do notice a little vibration, but it doesn't seem all that serious.
I was thinking of going with a mild turbo setup. Is it safe to boost?
I'm very skeptical of those all-in-one cure-all magic bottle treatments, but there appear to be some out there which are able to raise compression via cleaning. Some BG products, auto-rx, and Marvel Mystery Oil seem to come up a lot on various forums. Are any worth trying?