As said there should be no oil in the expansion tank, the obvious places are head gasket and modine leak.
It's quite possible to leak oil in water without affecting compression depending of course on where it's gone.
From your description of the water overflowing the cap, I would get a new cap, it may be localised boiling due to lack of pressure in system owing to duff cap seal.
It won't be bothered about those temps at all, the std fan switch is probably about the worst switch ford could have fitted, it just stays on far too long, they only need cycling a few degrees either way of stat temp, this leads to overloading/getting hot issues of the fan circuit.
It is fairly normal for the fan circuits to melt the fuse/holder and relay contacts in the fusebox, for safety rewire it with a decent standalone circuit.
One of the two most essential jobs to do on a cossie of any age is the fan circuit and the pump circuit.
What head gasket did you use, how many miles has it done etc.
tabetha