Sounds like you need to try a mild polish to start with. Megs 205 would be a good pic.

Im not sure Ive ever had oak tree stains so Im just guessing what would work but a polish would be something to try.

Or you could try this bug sponge with plenty of car soap, "safe scrub bug and tar sponge"
http://classic-motoring.stores.yahoo.net/saf-scrub.html