I dont think 15 is going to starve your cam at idle but 5 is way too low. The 15-40 is worth a try. Sounds like the engine is still tight and your oil is working harder and getting hotter than you might think. You can also try draining a quart of your 30 and adding quart of lucas oil stabilizer (some times it only takes a half quart) to keep your pressure up while you break it in.
