The oil is far more likely to be engine oil. The rear main seal on the crankshaft is a rope type wick and is notorious for leaking and a S**T to replace. The sump needs to come off, and in a Carrier, that means pulling the engine out
The design of the seal means it can't be replaced with a modern synthetic seal or be sleeved (Oh, Henry, what have you done?). When reinstalling the replacement rope seal, soak it in oil for 24 hrs beforehand to get it to swell and seal properly. Even then, expect some oil to leak. This is to ensure you don't get lazy and forget to regularly check and top-up your oil!