Weird lean symptoms?

I have been experiencing issues with my car running lean. When I bought the car, it had a misfire. so, I replaced spark plugs, distributor, spark plug wires, fuel filter, and a new o2 sensor. car started to run great, but I noticed it still never got good gas milage these cars average 40, I get maybe 20. so, after about 10k miles on the new ignition system I checked the spark plugs and they were showing signs of extreme lean conditions. With these lean conditions I’m experiencing gas in my oil. The car doesn't blow smoke, and it runs better cold than warm. I decided to check each cylinder after and noticed some decent pitting on each piston, but the cross hatch and cylinder walls seem to be in good shape. I should note the car has always thrown a p1320 primary ignition code before and after the parts and I'm just stumped as to what’s causing the lean mixture in the cylinder and pitting on the pistons and terrible fuel economy put the motor still runs fairly well no misses or noises and I want the car to last as it’s in great shape for a 98.