What would cause oil to get in the coolant of a 4.3 vortec? on 1998 Chevrolet S10

the most oil appears to be on the right side bank.

Asked by for the 1998 Chevrolet S10
OIL IN COOLANT. Defective oil cooler, it's part of the radiator! Any coolant in oil?
Does not appear to have coolant in the oil
Oil and trans. coolers can be tested to confirm. Get your mechanic to check it for you.
Thanks. I installed a new radiator before I noticed the oil in the coolant. I will get radiator checked. Is there anything else that could be the cause?
1 more answer , 1 more comment
maybe from old rad. flush system and see if it comes back. could also be leaking intake gasket
Thanks for info