maybe a stupid question but what exactly am I looking for with the check valve? I have not taken anything apart yet.
I would just make sure oil & gunk have not built up on it. Usually the check valve is just a ball and a weak spring.
I agree, you probably need to change your air filter if you haven't done that in a long time. Likewise make sure the belt is tight.
I would order new head gaskets and pull the head too, your reeds have probably encrusted with oil, they are easy to clean though. You might also find an issue with one of your reeds which might be causing the issue.
Refill with a NON-detergent, preferably synthetic compressor oil. Available at any big-box store, Amsoil online, or some auto-parts stores will carry it. Don't use regular automotive oil as it has detergents and the foaming will cause oil to reach the head, cook, and become a black crusty mess...