Golf IV / Jetta IV :: Oil Level And Blinking Check Engine Light

I just bought a 2000 GTI with the 1.8 turbo. It was very affordable, but it had a few problems. On hard rides, the check engine light was blinking AND I think there was too much oil in the crankcase (I say "think" because the dipstick funnel is gone and I'm not sure how far down the dipstick should go -- new one is on order).

I got rid of the extra oil, and now if I keep the oil in the hatched area of the dipstick (with the dipstick pushed down as far as it goes), the check engine light will never blink even under hard driving. But with the oil that low, the oil light will flash and beep on hard turns. I was experimenting, adding a little oil at a time, and hoping to find the magical sweet spot where neither the check engine light nor the oil light would go off. Instead, tonight I found the opposite -- the sour spot where BOTH of them go off.

So now I'm thinking maybe I need to change to a thicker oil. This car has 124,000 miles on it and was recently refilled with 0w-40. On my last car (a 1994 Golf with the 2.0 liter and 140,000 miles), the oil light would beep on hot days. I solved that permanently by changing from 0w-40 to Mobil 1 15w-50.

Finally, I checked the codes and it said there was a misfire in #2 cylinder as well as general misfiring So I checked the plugs and discovered that the previous idiot had gapped the #2 plug at 1/8 inch! So I (the present idiot) regapped them all and the engine seems to run better, but maybe going for months with such a huge gap has already fried the #2 coil. Should I just replace it? (or all of them?)