I would consider changing the oil cooler out, the Stillen one takes oil capacity from 3.5 litres to 4.5 As a precaution I have always added 0.5L every 500 miles and done a oil and filter change every 3500 miles.
My first 300ZX engine run out of oil after 3000 miles and threw a rod on me so learned my lesson the expensive way a long time ago
Been running this rad (with a few other engine upgrades :) ) and doing the oil as above for over 10 years and 40,000+ miles with no problems