-6 is WAY overkill on a feed. AN4 is MORE than plenty.
AN6 is way too small for a return. AN-8 is borderline if you have to, AN-10 is recommended. Also, its gotta go continually down, no traps like you have in a sink drain.
Reason being the oil goes in a liquid, comes out as a frothy slop, and it needs to drain freely.