Can someone answer this question? Is it true that once you switch to synthetic oil, its harmfull to go back to non synthetic? or is that a myth? I've heard that said so many times, but was never really sure of its validity?

Its just getting to hard to find 15/50 mobil 1, all the stores in my area are always out, its been that way for a year now. Plus the cost seems to be too high.

Can anyone recommend a good non synthetic easy to find oil for a high milage (165K) 92 400E?
Thanks

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