1999 century oil in coolant

Hi guys,as title says,i got some oil in coolant in my '99 century with detroit 60 series.I bought this truck in november last year,with 1023000 miles,now it has 1100000 miles on it,runs very good. Few months ago i had to rebuilt cylinder head,new injector cups,few more things...all together 4500$.Shortly after that,i noticed coolant leak from the radiator,so I replaced the radiator.Also replaced TRS and SRS sensors,because i had check engine light coming on occasionally,causing problems in engine performance.Now i have this problem with oil in coolant.I change oil every 10-12 000 miles,and truck does not consumes too much oil,like previous owner,I add 1/2 gallon or gallon after 8-9 thousand miles.Truck runs very smooth,engine is very quiet both while driving and on idle,and according to ifta for last quarter,my MPG is 7.8 and by my calculations its constantly 7 and above(im avoiding heavy loads though ).So,what could cause oil in coolant?Could it be bad oil cooler,some seals?Im not really into mechanic.