Which conventional motor oil has the least breakdown?

I have been using Valvoline for many years and am now finding that it seems to be breaking down so much easier now. I still use a conventional oil. I haven't made the switch to a synthetic yet but I am looking into it. So my question is regarding a conventional oil right now.Which conventional motor oil has the least breakdown?
it has always been a good oil everyone well have there own say about what they like better... after driving and working on cars over 35 yrs ive yet to find any diiferants between any of them and ive used pretty much used all of them if you think its not working for you just pick another and you could also go up a weight like from 5w30 to 10w30 might help ive always used 10w40 in everything i used synthetic since it came out any ive not seen any differants but i guess thats good still should change it more offen than not so changing more off well help Which conventional motor oil has the least breakdown?
i have used pennzoil in all of my cars for as long as i have driven without a problem.

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