Jack,
Many truck/tractor and industrial engines use water-type
oil coolers. These are usually seperate units (not in the radaitor) and have some kind of internal
oil thermostat.
However, if the radiator cooler is good for an automatic transmission, it might be OK for engine
oil.
Things to consider:
1) Capacity - Is the radaitor cooler large enough to do any good? Air-type oil coolers operate with larger differential temperatures between engine oil and ambient air temps.
2) The radaitor temperature is not directly regulated. The engine's thermostat maintains the engine's temperature only. At times, the radaitor temperature may be significantly different (ie.. during engine warmup - very little coolant circulating through the radaitor).
3) Is the radaitor cooler and lines running to it able to withstand engine oil pressures?
4) Will the radaitor cooler clog or corrode with "dirtier" engine oil flowing through it?
There seems to be allot of unknowns...
Mike D.