That Robinair pump doesn’t seem to have an oil mist filter available or even a way to attach one. I have one that’s been sitting around that I have not really used for anything because of that reason. The exhaust flange is weird and I don’t see how anything would attach to it. Do you happen to (or have in the past) use that particular pump?
EDIT: Actually, never mind. I think I just found a solution:
https://www.instagram.com/p/BRUM-IahrV4/?taken-by=henryholsters