The issue with this is that there's no thermostatic control on the coolant into the oil/water heat exchanger. Typical
ΔT across the radiator is around 10°C, so a 95°C engine outlet temperature may be 85°C at the engine inlet, where the coolant for the oil/water exchanger is sourced.
Even at 85°C coolant inlet temperature into the stock oil/water exchanger, it does a really good job of keeping the oil into the engine at sub-100°C when driving on a 30°C day. If you drop the coolant temperature into the oil/water exchanger to something considerably lower by passing it through a sub-radiator, before it enters the oil/water exchanger, you can end up with an over-cooling issue, where the oil runs at a sub-optimal temperature under normal driving conditions, as well as taking an extended time to reach operating temperature.
Where competition use is concerned (heat removal aspect), oil/water heat exchangers are far superior in heat transfer compared to oil/air heat exchangers (all things being equal). The drawback is that the system adds weight compared to an oil/air system, adds complexity, and is more expensive. The seven-row, or even stock 5-row stock heat exchanger has the potential to be far superior to any external oil/air cooler, depending on how cool you can get the coolant supply. If the stock system running a coolant temperature of 100°C can keep the oil to sub-130°C on a single lap of the Nurburgring, imagine what it could do if you lowered the coolant supply to 60°C. I know for a fact that the G16E in the Rally 2 Yaris uses an oil/water heat exchanger for oil cooling.
Concerning cooling the water, something to keep in mind is that while water has a higher specific heat capacity than air (4.23X), it's also almost double that of oil, so while it absorbs heat much more readily from the oil, it will take a larger radiator to reject all that extra heat to atmosphere than directly cooling the oil with air. Some additional benefits are no pressure drop associated with oil/air systems, and no potentially catastrophic point of failure. Meaning if an oil line running at 6 bar bursts, it may damage the engine very quickly, where if a coolant line running at 1.1 bar bursts, there's quite a bit of time before damage from overheating would occur.
I really hope that was comprehensible. I tend to get carried away. I know what I mean, but often fail to get it across in writing.
What's happening with my Yaris is placing the sub radiator on the stock oil/water exchanger discharge. Maintaining the warming effect of the engine coolant into the exchanger, but rejecting the absorbed heat from the oil to atmosphere before it reenters the main radiator. Effectively "removing" oil heat (which is significant) from the main cooling system. I'm not circuit driving, so not looking for a large change. My system adds .75L of coolant to the system. I've done the math and know what the change in heat should be, but actual results will be interesting once I start testing.
