Anyone using a plate heat exchanger for a oil cooler opposed to the traditional tubular style cooler. If so how big of a cooler would I use. I know they are more efficient than the tubular style. I am looking at a 12" X 5" 20 plate with 1 1/4 tapings 316 SS. Would that be large enough?????

I would certainly think that is big enough. I use the plate coolers quite a bit. I use the ones that go in the bellhousing of a Speedmaster type set up. I don't think they are dimensionally that large. I'm not sure of the exact measurements of the top of my head, but they are not as big as your 12 x 5 I don't think. I use a 13 or 15 plate, depending on the power. I think you will be just fine with what you have.
Eddie