I use the manual's recommendations as the max amount of time before I change the oil. I check the oil each time I start the engine. When the oil gets real dirty I change it even if it is way before the recommended time. There have been times I changed the oil 3 times before the recommended time. As far as using synthetic, again, it all depends on what the manual says. I've been using 10w-30 for as long as I can remember and have never had an engine blow up or die. Maybe its because I change the oil when it needs it.

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