Oil in radiator

Is it engine oil, or transmission fluid? The transmission oil cooler is in the radiator, and if it has a leak, the cooler pressure is higher than the cooling system pressure, and forces oil into the radiator. The same applies if it has an engine oil cooler in the radiator, not all vehicles do. Another possibility is a leaking head gasket, or a crack in the block or head, it would have to leak in a specific location near a pressurized oil passage.

Pressure in the air conditioning/heating system, are you talking about refrigerant pressure, or cooling system pressure? By 'super high' what do you mean? If it is cooling system pressure, the cap should let off at 15-16 psi. If you checked for a cracked block, was the method used able to rule out blown head gasket? If it was a block test for hydrocarbons in the cooling system, if the leak is not in a combustion chamber(or near), you may not have a positive result. Are you confident that it is engine oil, and not transmission oil, or does it have an engine oil cooler integral to the radiator? You only have a few possibilities in this case, either oil was put in the cooling system physically, it is transmission oil from a leaking cooler in the radiator, it is engine oil from a leaking oil cooler in the radiator(if equipped), or it is a mechanical engine condition where pressurized oil is passing into the cooling system.

It would be advised to have it looked at. Realistically, there are only a few possibilities. If for one, it has an automatic transmission, it has a cooler in the radiator. Even if it has an auxiliary cooler in front of the radiator, it also flows through the radiator cooler. An auxiliary could have been added as an additional for towing. The only other means of oil/coolant interaction is with an engine oil cooler in the radiator, or internal engine architecture. Hope I have been some help.