[Sephitrask]

Why are you wanting to install a secondary radiator? Are you having cooling issues now? If so the issues causing that needs to be remedied before any modifications are done. On to the answers to your questions:

Oil temps do not turn on the fans, coolant temps do, and they turn on at around 210 F. Rx8performance.com sells a relay kit that turns them on at 185 F, I have that on my car. For the oil temp sender I found it best to get a modified banjo bolt that you can screw the sender into and us it to replace the banjo bolt either on the front cover or drivers side oil cooler (put it on the side pre-cooler not after) You can get an oil filter sandwich adapter but I found that the oil filter location made the thing a bitch to deal with.. I ended up throwing mine across the driveway in frustration and ordered the before mentioned banjo bolts. Again, Scott at Rx8performance.com made those for me.

All in all the cooling system on the rx8 is very good if functioning properly, unless you live in extreme heat or track the car it should be fine. Do you have a undertray that is intact? it is an essential part of the cooling system