In theory the gaskets should be oiltight, practice is different though.
I have always used LOCTITE 5900 or 5910 flange sealant, pricey but good.
I have used this on all manner of gaskets, never once had a leak, and have used this on std sump gaskets and also with NO gasket at all, still no leaks.
I once had to take off a sump that had been put on with this, some weeks earlier and spent 1 hour straightening the sump flange as it had to be levered off, it di not leak a drop however, used for temp seals on fuel tanks stat housings etc etc, doesn't fully harden ever, excess can simply be pulled off when cured, which is 5 minutes with 5910 or around 1 hour with 5900.
I have also used it to stick down carpet in house, on gutters, hosepipes, trim on cars.
tabetha