OK, it's quite possible that I'm missing something here:

I go to check the air filter on my 91 GS500 (just got it last week) and discover the airbox has OIL in it.

I don't mean a 'little' oil but a freakin' LOT of oil - like almost going into the carbs high.

Am I missing something here???

Have you overfilled with oil at some point?

The breather hose from the engine head to the airbox can leak oil into the airbox if oil levels are too high.

I was thinking the same thing.

Or was the bike crashed/tipped over for a period of time?

The cam chain tunnel (AKA the "hellhole" for bolts and tools) is where oil to the head drains back down into the crankcase. If the bike is laid over on its side or even upside down for any length of time, quite alot of oil could wind up in the head, and that oil will flow out the crank case breather and into the airbox.

Thats my guess how it got in there. You would have to overfill the case ALOT to get anything more than the usual oil in the airbox otherwise.

Quote from: JamesGOr was the bike tipped over for a period of time?

That was my guess.

You might want to check the drain tube and make sure it can actually drain...  I had oil coming out of mine a little while ago.

Here 's the answer to my question - the drain hose IS pinched tight.

Quote from: gsmetalHere 's the answer to my question - the drain hose IS pinched tight.

YAY!!  I win!   :mrgreen:

Hopefully the oil you had in the airbox took a really long time to gather...  If not, then you have more investigating to find the source of the oil.