Might be a long shot but on series 2's they have an oil return drain bolted to the block next to the main cap. This helps drop the oil further into the sump. Now (ive seen this happen) if you put a new sump on which isnt a S2 item (non turbo ones dont have the hole in the baffle plate) then the baffle blocks this pipe up and dont let the oil drain quickly enough into the sump from the turbo and will cause the symptons you state.

But in all honesty its probably rings!