You can switch between synthetic and non-synthetic oils anytime you want. From what I've heard, seen, experienced, and READ it causes no harm. As a matter of fact if your using synthetic and are low on oil you can just add it even if it's not synthetic. The mixer will just delute the synthetic making it less protective. As for using and needing to stick to a Mobil brand that's just not true. As for using the non-synthetic oil in the winter to save $$$ I understand but I wouldn't change to a non-synthetic oil just to a cheaper brand of full synthetic oil. I'm using Amsoil 20W50 and in the winter if I do get around to change it I will use what ever is on sale at the time from my local parts store. Quaker State synthetic usually $3/qrt, pennzoil $3, Castrol $3.50 there all cheaper than $6.50/qrt for Amsoil.
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