Ford 3400 Tractor Oil in Water, No Water in Oil

A friend of my Dad's has a Ford 3400 Diesel tractor (painted yellow, maybe an Industrial?) tractor that is getting Oil into the Radiator, but NOT water into the oil. What on this tractor would be the cause of this? Do they have an oil cooler in the radiator?

Thanks

Is it a Select O Speed?
The SOS tractors had an oil cooler in the bottom of the radiator.
Never heard of one of those failing but they could.

Had to double check, but it is standard. So what would cause oil to get into the radiator, but no water into the Oil?

If it is a diesel these 3 cylinder engines are known for cavitation. Vibration causes bubbles in the coolant to wear away the passages. Perforation into the cylinder walls is fairly common. Less common but still not unheard of is cavitation in the lower end of the water jacket where there is an oil galley. I don't know exactly where they rust through but the result is oil in the water. There is an old school machinest here who fixes them by running a reamer into the oil passage and then epoxying a steel tube into the bore.
Again, I can't say where he approaches it but he has fixed many of those blocks that way.
The sad thing is he has cancer and I'm afraid we're going to lose him.
If you are certain the oil is not just residue floating on top of the coolant then that is likely the problem.
Means pulling and disassembling the engine of course. Then you could plug all holes, pour soapy water into the oil galley and apply air pressure to your system.
You would be able to ascertain where it is leaking by the bubbles.
If your engine is a gasser then I don't know. They were not known to have the cavitation problems.
Maybe someone else here has ideas.
FWIW, if you do need a block both gas and diesel blocks are identical so either will do.
Where are you located?
I have blocks here that I almost can't give away.

Yes its a 3 cyl dieseal. Saddly the tractor is West of Phoenix, Az.