If you mean a water-to-oil heat exchanger like the stock "oil cooler", just with a higher capacity, Dali Racing has one on their website: http://www.daliracing.com/v666-5/cat...cfm?focus=1134. However, after testing it Mark told me that it helped but that it wasn’t large enough to work well if you are constantly running between 7-8k rpm.
Maybe the OEM water-to-oil heat exchanger in some existing car has enough capacity to cool an NSX’s engine oil. BMW M5s of the 400 hp E39 generation had water-to-oil heat exchangers for the engine oil and maybe you could adapt one of those to work in an NSX. As a note, for the 500 hp E60 generation, BMW decided to go with an air-to-oil cooler instead.