I can't use the thermostat thats on the car already?

No. Thats for coolant.

You already have a thermostatic oil sandwich plate?

ic, thanks!

nope.

How hard it is to install myself?

Very DIYable. Take off the bumper, drain the oil, put the sandwich plate on, mount the core, run the lines and refill with oil.

^^^^
What he said

Does anybody know if there is another aftermarket Thermostat different from Mishimoto's?

The nismo de if you plug the extra coolant pipe, the 08 g37 lower temp one, or the de ones are all different temps to use a de one you just have to plug the second nipple on the de thermostat. It is double acting thermostat so it is a bitch to make hence no one makes one. when it gets hot it both opens the draw hose and closes the bypass tube at the same time.