Just bought 2010 Int prostar with ISX 435 with 450k miles on it. 1st 3k mile I've added 2.5 gals of oil, the next 3k I've had to add 1gal. It originally had 15w40 oil and when I serviced it I had 15W50 synthetic blend (mystic) put back in. Could that possibility be the problem? No oil leaks by the way. Thanks in advance for your help.