Hmmm....my first thought is that you might be looking at this wrong.
Use a radiator to cool your Water
Use an
Oil Cooler to cool your
Oil.
For me, the 2 temps can vary drastically from each other. I use a manual by-pass valve which allows me to turn the
oil cooler on or off. I find that during general driving, even without the oil cooler, my oil tends to run a bit colder than I would like....but my water is fine. While driving sporty with high engine revs, My oil will quickly race up from about 170F to 240F without the cooler....and again the water temp stays in the normal range.
The point is that in my opiniom, it might be possible to have Dangerously high oil temps with very cold water temps...and visa versa.
So if your radiator cooling capacity is too low...you need a bigger radiator...not more oil cooling. This way you don't make icy cold oil while trying to compensate for excessively hot water.
Just my thoughts..
Have fun!!
-Rob