Old guy at the parts store told me to ditch synthetic oil in my '66 Mustang

I've been running synthetic for years cause everyone says it's better but he said these old engines need the zinc and seal swellers in conventional. Swapped to a high-zinc 10W-30 last month and the lifter tick I've had for 2 years is completely gone now. Anybody else been told to go backwards on oil choice?

2 comments

2 Comments

oliver2421d agoTop Commenter

Buddy of mine ran synthetic in his '70 C10 for years and wondered why it always smoked on startup. Switched to a conventional 20W-50 with high zinc and it cleared right up after a couple oil changes. Said the seals were probably too dried out for the slick synthetic stuff. His engine's a stock 350 that's never been rebuilt so that old timer at the parts store probably knew what he was talking about.

eva_lewis1d ago

Same thing happened with my old '68 F100. I had been running synthetic for a few years and it started puffing blue smoke on cold mornings bad enough to embarrass me at the stoplight. Switched to a conventional 15W-40 with high zinc additive from the tractor supply store and it stopped after about three oil changes. @oliver242 your buddy's experience lines up with what I saw, those old seals just need the thicker oil to stay plump.