Sounds like the lower o-ring seal is starting to go. You can get away with running the machine like that for a few years if the leak is slowly seeping.
There's no way to check or add oil without removing the inner/outer tub and cracking the trans open. If you go that far, might as well remove and disassemble the trans, replace the o-ring, and fill it up with fresh oil.
It's probably better to do the repair in the not too distant future if you want to keep the machine. Whirlpool has pretty much discontinued just about everything for these machines and the parts that are floating around out there are pretty much all that is left for us to use. To top it off, most appliance parts outlets have been throwing away the old parts to make room for the newer model machines. It's this reason I've been aggressively stock piling parts the last couple of years.