what does it mean, three plugs were fine, and one was wet with oil, threads and all. car is running great. can I keep going without big catastrophy??

Worn valve guides ....

Worn rings .....

Worst that may happen is fouling the plug and having no fire on the commensurate cylinder.

what is the opinion of that small can of oil additive "compression in a can"
couldnt hurt right?
im runnin 20 50 cheap stuff now.

I wouldn't think it would hurt anything.