My 2013 3500 SRW Aisin has had synthetic 5w40 since the first change and now has 70K miles. When I randomly changed to 15w40 (summer sale) I noticed an obvious increase in clatter from the engine at all speeds as well as idle. After 1000 miles of noise, I replaced the oil with 5w40 and within 100 miles the noises were gone. What would cause the increase in engine clatter. I really wanted to run 15w40 for summer but am hesitant to try again. FWIW, the 5w40 is Valvoline Synthetic and the 15w40 was the new T6.