The stock oil pan has a block-off plate for the oil cooler lines that would have been installed for such heavy duty usage.
I also took the liberty of finding a low profile cooler manifold for that location. It says it would fit the G8.
Edit: I had to go back and edit because I realized I overlooked an important question.
Are you on DOD? Or did you do a DOD Delete while blocking off the relief valve port in the oil pan? It is important, as higher viscosity oil may just end up discharging through the relief valve, which may cause problems with higher viscosity at operating temperature.
It's more of a disclaimer for me, because while the oil will be thinner at operating temps, pressure will want to increase. Having a plug in there will allow for the increase without sacrificing flow.
If you are DOD Deleted and plugged the relief port, then you should consider something other than 5W30 Oil. Try stepping up your oil viscosity to a 5W40, or even 5W50 for racing applications.
It will handle shearing forces
better than 5W30, and you won't have any problems at startup getting oil where it needs to go as the viscosity will remain the same as any other oil rated SAE 5 at startup temperature.
That alone should kick your oil temps down some. The oil film thins out at high RPMs due to the aforementioned shearing forces
, and the SAE 40 or 50 rating at operating temp will help alleviate some of that. (BITOG has good reading on this subject).
Installing a cooler may hurt your ability to get up to temp during normal driving, though.
I'm not sure what to tell you about your coolant loop. Assuming everything is in order, your Radiator needs to be able to handle more heat. I wonder, though, if something is wrong. That, I cannot say....