Is it bad to alternate frequently between synthetic and dino oil?

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Oct 4, 2002
The myth that you shouldn't switch back to dino after using synthetic has largely been debunked. But is there any issue with switching frequently between synthetic and dino oil? And would using synthetic only every 3rd or 4th oil change be worth it, or is it not worth it unless you use synthetic exclusively. These are hypothetical questions, but came about because I can change the oil in my cousin's Lexus every now and then with my stock of Mobil 1. But it usually gets Valvoline dino oil changes at the dealer.
There is no hard evidence that I know of on this: But switching oils with between a base of conventional oil and Group IV might cause problems with the main seal as it adjusts to the differentlils. The other issue is that if your cousin doesn't want to take the initiative to use Mobil 1 why should you care. [I dont know] I see little value in using Mobil 1 periodically. [Smile]
But what about switching between winter and summer? Would it not be of benefit to use a good synthetic with very good low temp flow in the winter and then use good non-synthetic in the mild summer? If it is mild, of course [Smile] [Confused]
Originally posted by kreativ: But is there any issue with switching frequently between synthetic and dino oil?
I guess I'll be one to say that there could be some potential problems. IMO when changing I would do so once or twice but it would be better to flush first, and maintain whatever oil is there. Oils can compete, as well as their additive packages, for the metal surfaces and perhaps dirt, for me, I wouldnt wanna keep changing to have the new oil try to remove the oil previous to it (which can be seen IMHO to that new oil as an contamination issue).
Originally posted by pepper32: I have been using Mobil1 winters and Pennzoil summers. I hope I'm not doing damage to my engines.
your not
I have, a couple now, boats where I run synthetic oil during the summer season, redline 10w40. But when I winterize them and lay them up over the winter for 6 months I use cheap dino oil. Been doing this for 8 or 9 years on my 351 V-8, that was a factory new replacement motor in 1993. No oil leaks or oil usage with it at all. And I usually run it on the winter dino oil for 1-2 hours the following season before changing it out to synthetic for the season. And before I started using redline oils, only since like 2000, I was using castrol syntec, pennnzoil synthetic and mobil-1 in it. So I would have to say, no, switching back and forth does no harm.
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