Hey Jaghoss,
It's tricky to diagnose a dry crumbly cheese without some ph readings before salting. Most likely, your cheddar's ph bottomed out during pressing. Maybe left the drained resting curd too long. I'm Canadian, but still an aspiring cheddar maker, lol... I'm looking forward to what other board users figure what the oily substance may be ... I've seen leaky cheeses under wax before but it was whey... All the best