The OEM valve covers are unequaled in design as far as preventing oil ingestion. Look at how they have complete baffles the entire bottom side to keep oil from slinging up and out like the aftermarket ones. And then the coalescing and condensing chambers and screens. The super low profile also do not allow enough room for the rocker arms to avoid windage caused parasitic loss from oil splashing back onto them.
Weapon X has the best baffled aftermarket covers, but again, none come close to the OEM design that is GM's best design to date addressing most of the past issues:
https://hosting.photobucket.com/albu...psotnsc4hz.jpg
https://hosting.photobucket.com/albu...pswyozynp9.jpg
https://hosting.photobucket.com/albu...psinvypjcz.jpg
So if you're dead set on aftermarket covers, make sure they have full length baffles. Will save you headaches in the future.