I have an
oil cooler and I have seen it work. Usually in traffic stop and go or on very hot days driving hard. In the fall it takes alittle longer to warm up the
oil but with the right
oil weight it shouldn't be an issue. I am still running 10w30 oil with great oil pressure. I have never blown or drained the oil out of the cooler when doing an oil change. I doubt its more then a quart or two in the lines and cooler. We change out oils way more often then most so it will get flushed eventually. IF your concerned about cool weather then just cut a block out plate to install.
I have a 99 Viper ACR. That car came with an oil cooler installed factory. The difference between the GTS and the ACR among other things was that that the filter on the ACR had a bypass valve installed that allowed oil to always go to the cooler regardless. The idea was to warm the car up until the oil pressure and coolant was up to temp before driving hard.