I've always tried to use one brand/formula of oil and stick with it throughout the vehicle's life. Is this stupid? Will it hurt anything at all to switch oils after draining the old stuff? Also, I've posted some other questions concerning a 6.0 Chevy gas engine. As stated in those questions, I'm currently using Series 2000 0-30W oil. I live in Montana, so I picked this oil because we get a lot of cold weather. Thanks in advance!