in that link, thats the oil cap, not the rad cap. thats from coolant getting into the oil.
its almost impossible to get oil into the coolant, unless you have a oil cooler in your car hooked up inside the radiator. not likely.
trans fluid is possible because we know you have a trans cooler in the rad. so trans and coolant can swap if a cooler is broken. coolant in the trans, or tranny fluid in the rad.
if your rad cap is gunked up, id think its dex coolant sludge build up.
and when was your last oil change, after 4 or 5,000 miles the oil breaks down, and the engine will eat it. no smoke or nothing.
id do a coolant system flush before the lim job, then drain it and do the lim gaskets, then add new coolant after your done with all the gaskets.
there is nothing you can performance wise while the lim is off except for porting the supercharger.