Hi, I recently bought an AS168S compressor (3L tank and cooling fan).
I'm guessing a simple typo and you mean AS186S.
If so, they don't use oil so forget about that. They aren't exactly known for reliability though. They used to be great, and I know people that have had them for years without issue, but with everyone and his grandmother re-manufacturing them they can be hit & miss these days.
Mine lasted 6 months, during which I stripped and rebuilt it twice.
One common issue is moisture getting in to the pressure shut off valve in the head, which then corrodes enough for it to stick, causing all kinds of funky behaviour.
Another is manufacturing gunk in the chamber itself, blocking the reed valve or causing it not to close properly. Remove the head and clean out anything that's in there.
These, and other issues are covered quite well by YouTube videos
I eventually gave up and went overboard with an Iwata Power Jet Plus. The air delivery is like night & day compared to the cheapy, but there are many more budget friendly options from either Iwata themselves or Sparmax, which are part of the same company.