iggman,
The blends/Grades; 5w30,0w30,10w40 etc... are the weights of the oils at different tempatures. For example in colder weather you want an oil that flows at real cold temps you would use the 0w30. In hotter weather you would use a 10w40. The reason the weights are getting thinner is also because the tolerances in newer engines are tighter. So the oil doesn't need to be thicker in order to provide that film protection on your cylinders, cams and bearings and inside your engine it needs to flow better with those tighter tolerances the manufactures make. As far as mixing the synthetic with petroleum base it won't hurt any thing at all. But like crewzin said. Why switch back and forth. You could just change the Grade of synthetic to meet your tempeture conditions. NOW if your running a turbo that is bad. Even if you change brands you can sometimes lose your turbo bearing. Grades are okay though. Now brands are also significant as to where they come from, ie some have a high sulfer content; Penzoil, Kendall, Quaker State mostly from the Northeast. Then you have a Texas grade like Havoline, Chevron, Mobile, these have a lower sulfer content (Don't get dirty as fast basically). I have done my research and the web can help you allot with info too.
Gman
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