any leaks from the rocker cover or head will end up on that vacuum can.
tightening the rocker cover screws is often a waste of time. take it off and check it is sealed properly and the gasket has not gone hard.
toyota originally used sealant only on he corners around the humps, but we always use the smallest amount along all its edges.
also look carefully down the plug tubes for signs that the internal seals are leaking

+3 that's engine oil, I would put the car up on a hoist if you can get access to one and degrease and wash the whole motor, once its clean it will be very easy to see where it is coming from.

Thanks for the replies.
Can anyone recommend a (reasonable) auto shop in the South Eastern ( Dingley Village) area of Melbourne to look see for me.