Radiator fluid getting into oil changed radiator no visible gaskets on intake or head leak no change in preformance other than loss of radiator fluid has engine cooler to radiator and trans cooler to radiator no visible leak around water pump heater works no fluid smell in passenger compartment

indicates blown head gasket
do a compression test to confirm then replace head gaskets and have the head/s shaved as they will be warped

Sounds like a warped head take to a mechanic have a compression test done. DO NOT USE ANY GASKET SEALER

This is common issue early sign head gaskets failure for EJ251 SOHC engine for external coolant leak. Second gen 96-99 with EJ25D is worst because when head gaskets failed, it's an internal leak that allows coolant and oil mix together in the combustion chamber to destroy engine.
You can drive it for at least 6 months or more as long as you keep top up coolant since the leak should be small at a slower rate leak. I drove mine for a while before change head gaskets. You're looking at `$1000-1200 job a dealership since you might as well change the timing belt, water pump, idlers while engine is pulled to replace head gaskets.

I am assuming its motor oil and not transmission fluid, right ?
You can have a leaking intake gasket or head gasket and get oil in the coolant without getting coolant in the crankcase.
Same thing opposite. The bigger the leak, the better the chance of getting both at the same time.

Oil in the radiator might be automatic transmission fluid cooling lines that run into the radiator that are leaking. OR its stuff that is collected over time if the cooling system was never flushed and cleaned out. Its usually antifreeze in OIL not the other way around which means blown head gasket or leaking intake gaskets.

Did you listen to the radiator too see if it gurgles when filling? When filling the gurgling tells you there are no air or coolant leaks in the engine much the same as an upside down soda bottle gurgles when turned upside down. Also look in the radiator too see if bubbles are coming from the bottom of the radiator while running at normal operating temperature with the cap off.? If you see bubbles coming from the bottom, you have a leaking head gasket. This can and will happen without the coolant leaking into oil, and without oil leaking into coolant. The gasket leaks right near the cylinder and sucks the coolant into the cylinder on the intake stroke, and blows it back into the radiator on compression stroke. This is why the coolant bubbles up from the bottom. The coolant that gets sucked in gets vaporized and comes out the exhaust. Loosing coolant ?? This is where it is going if no leaks under or around around the vehicle.Good luck I hope this solves your problem. Please post a solved comment on fixya, so I receive credit for the solve.

If there is no oil cooler inside the radiator, the most common cause is a leaking intake or head gasket.

SOUND LIKE HEAD GASKET LEAK.CHECK ENGINE OIL FOR SIGNS ANTIFREEZE CONTAMINATION..OIL WILL LOOK LIKE MILK SHAKE.IF USING WATER IT EVAPORATES AT 212 DEGREES. ENGINE COOLANT TEMPERATURE CAN CLIMB UP TO 220 DEGREES.IF OIL IN CRANK CASE LOOKS GOOD.ADD 50/50 WATER AND ANTIFREEZE. ANTIFREEZE WILL RAISE BOILING POINT TO KEEP COOLANT FROM EVAPORATING.SEE IF YOU LOSING WATER ITS BOILING AWAY LIKE WATER IN A TEA KETTLE.

First thing that pops into my head is a leaking intake gasket. Second is bad head gasket. Third is cracked head or engine block.

This is probably a head gasket problem and not an oil cooler problem.

The oil cooler is fairly easy to test.

Remove the cooler from the engine but do not disconnect cooling lines and then pressurize the cooling system. You can use a radiator test tool to put pressure on the cooling system and look for leaks. If it leaks, replace it.

You may also be able to bypass the oil cooler altogether if it leaks.

The head gasket is another story.

Get a 1/4" pipe to spark plug fitting and put a male air coupling fitting in it.

Then you can charge the cylinders with air from your compressor.

Make sure each cylinder that you test is at top dead center so that the valves are closed.

If air bubbles into your coolant, you found your problem.

If air leaks out your intake you have a bad intake valve

If air leaks out your exhaust you have a bad exhaust valve etc.


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Tim