Use Permatex non-hardening thread sealant (or Form-a-Gasket, old #2) - definitely oil resistant, I've always used it to seal head bolts, never failed me yet in 50 years of engine rebuilding. The only thing I dislike about it - don't get it on your hands or clothes (or anything else you don't want stained), it's a real pain to get off of you once it gets there. Great stuff -
Regards,
Steve