Made a discovery -
In the wagons with the annoying power shoulder belts. If you leave the driver's door slightly open, the control system draws 160mA indefinitely, draining the battery over the course of a few days. Just another reason to replace them with manual ones.
Okay that confirms what I was thinking. There are no signs of water in the oil so I'm thinking it must be coming from the water pump and just appears as though it's coming from the oil pump. Every single one of the water pump bolts was so rusted they broke off except one so I'm sure it needed replacing. I have a new one ordered once I get it on I'll let you know what happens.... Thanks again