Incorrect, full synthetic has higher zddp (zinc) than conventional which is what older engines require a lot of because of metal to metal contact. TBI engines need this even more so because of our flat tappet cams. What likely happened is the synthetic oil cleaned up your dirty engine making existing issues more obvious or apparent to you. Switching to synthetic will still increase the life and efficiency of your engine even with your issue.
And yes oil does wear out its proven by science.
I think your incorrect on the part of synthetic oils having more ZDDP (Zinc) in the oils.
If you look up on Google about the ZDDP add packs in todays oils its down allllllll across the board even more so in synthetics being that most cars today are roller cams than the flat tappets from the TBI engines. Alot of oil guys an engine builders say you need at least 1200 PPM of zinc for you to have protection against wear in a flat tappet cam engine.
While if you look up most oils including alot of synthetics have around 800PPM of zinc. As of now they're trying to lower that down to 500 if I recall being that back in the day zinc would clog catalytic converters thus the reason an switch to less zinc oil. Now dont get me wrong theirs really good synthetics out there that have the required zinc such as AmsOil, joe gibbs, brad penn, an Valvoline VR1 racing oil though I read they lowered the PPM of zinc but have to make sure. But yes other that the VR1 all the other oils have good amounts of zinc.
Now other synthetics such as regular mobil 1 or casteroil syn an what not dont have the required enough zinc. You'll deff need to run a ZDDP additive.
Now heres where theirs an exception the ZDDP really is needed in new flat tappet engines that are being broken in today. Still need to use ZDDP the rest of the engines life but since lets say our TBI trucks were built at a time when all oils had enough zinc our trucks were broken in an ran with that old oil just until recently so nothing really to worry about with running todays oil in our old trucks. Wouldn't hurt to run a ZDDP additive for that protection too.
Back to the whole synthetic vs conventional debate. The OP asked about oils an really any good synthetic is good to run. I tend to choose mobil 1 or if you want AmsOil or bradd penn if you want longer intervals. Basically really I stick to old school, check your oil regularly an change every 3000 and defffff put a QUALITY oil filter!!! Doesn't matter what kinda oil you get if you got a bad filter its gonna get dirty.
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