The newer bike have oil coolers can I fit one on my '96 GS, for better engine cooling and performance maybe?

The newer bikes have fairings and need them. I can be done but you will need to figure out what plumbing has been added to the motor, both internally and externally.

If you install a cooler, the mostly likely thing to happen would be oil leaks. People ride the naked GS hot weather all around the US with no problems. It certainly won't do anything for performance that I know of.

well figure you can tap the lower gally, using a restrictor oriface inline, into the cooler and then dump into a non pressureized cavity,  oil pan would be ok and the cooler oil can go right to the pump.