I thought the oil cooler Stillen uses is an Earl's 19 row oil cooler . What brand is your's? I believe Earl's oil cooler is a Setrab oil cooler which should have very little drop in pressure as long as you use #10 size hose. Are you using the oil restrictor from the stock return hose?
Is your oil cooler upgrade a full flow oil cooler circuit or a stock setup oil cooling circuit? The stock setup uses a small portion of the oil flow to flow through the cooler and back to the pan instead of to the engine. I don't know of any other OEM oil cooling system that does it this way. Why not offer a full flow oil circuit system using a sandwich adapter to the oil filter. You will have to plug the oil filter tree for the stock oil cooler output. If you go with full flow circuit, you will want to use an oil thermostat so that only hot oil flows to the cooler. Perma cool sells one that opens at 185 degrees F.