Ultra Gray or Ultra Black RTV

I prefer the black for that application because it's more pliable and soft after it's cured, the grey I like more for high-pressure sealing like around water ports and thermostat housing because it gets really stiff when it cures.

FWIW I smeared a thick bead of Ultra Grey to seal the baffle I welded into my stock oil pan, 25k miles of being submerged in motor oil and there's been no degradation at all... I think it's safe to say the grey is plenty resistant to oil. I thought about redoing it before putting it on my next engine but that stuff still has no interest in coming off, it's stuck in there tight.