CAT C13 KCB, oil in water after inframe

hey ya'll,
i just recently inframed a cat c13 acert kcb, everything went well and it started up great and ran great no lights or codes, its done about 5,000 miles already and the driver said it felt better than before, however he noted a couple days ago there was layer of oil sittin on top of the coolant in the reservoir and since then his power has decreased also. im not sure as to what could be doing it, liners are new, head is rebuilt, only thought is an oil cooler going out but how likely is that right after the inframe, but major components do go out after rebuilds. i did this one myself and cant think back of doing anything incorrectly. what are things i should be looking for? any opinions on what it is? hopefully its nothing internal and major thanks!

 

yeah its oil, transcooler is on back of tranny, coppers are new with the head, ya think oil cooler?

 

ya think i could just bypass it? or is it necessary on an Acert?

 

okie, an oil cooler wouldn't cause loss of power right? unless its something internal?