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Bobupndown
What could possibly go wrong?
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Can't imagine it will affect an old school engine too much. Topped my old Mini up with 10w40 when I didn't have any 20w50. Any oil better than none being my theory. If you're concerned give it an oil change.
Don't think a modern car would appreciate a slug of 20w50 though.
Don't think a modern car would appreciate a slug of 20w50 though.
ajlelectronics
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Synthetic + mineral = semi synthetic. Panic ye not, might even be of benefit in cold weather. You have the Zinc from the Classic oil anyway.
barracuda816
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It won’t cause it any issues what so ever. 
Not leaks (well till it'd gone way bad anyway).
Info I had was that the much better performance of the high-spec synthetic over the old-school, lower spe mineral oil (something to do with long-chain polymers or suchlike) is that the oil is too good for the engine design - insomuch as it stopped the oil scraper rings on the pistons from doing their job properly.
This leads to bore polishing, on to piston blow-by, loss of compression, crankcase pressurisation (and oil seal leaks) and oil consumption and carboning up of valves etc.... so you get an oily, smoky, reduced-performance engine.
When I worked for a Castrol Oils Distributor many years ago - their technical support dept aways advised you recommend the specification of oil the Mfr recommends (or Castrol equivalent) that the engine was designed to run on, and not any "better", higher-spec oil - for the above reasons.
I'd use the recommended oil and change it more frequently over up-speccing the oil personally (as I figured Castrol Boffins knew a lot more than I did!)
rcx132
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I hope I don't offend, I just see this as a discussion and I'm no expert. As much as your explanation sounds good, it actually lacks the crucial bit: "it stops the oil scraper rings on the pistons from doing their job properly" because ... [explain the process that no longer works with synthetic oil].
If the oil is of the same viscosity that the engine was designed for, then what's changed from the engines perspective? The oil lasts much longer, well that's only a good thing. The oil has slightly better lubricity... maybe this could have some unexpected result, but does it? How significant is it? Does it offset the massive benefits from fully synthetic?
Old oils are c**p. Project Farm and other channels tested some of them against modern spec oils. The technology has advanced. Mobil 1 do (last time I checked) recommend fully synthetic for older cars. So does ChatGPT. If Castrol don't it could just be that they're playing safe.
ajlelectronics
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OHV engines (apparently) require Zinc additives which are missing from modern synthetics. I have no opinion on the matter, having never looked into it much. However it is interesting that the 12v 2.9i in the Scorpio is specced for 5W30. Only difference over the earlier Granada engine is hydraulic lifters and O2 sensors. That was specced for 10W40 from memory.
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Morrisman
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I run my BM motorcycle on 20/50 dinosaur juice. Heading off to Europe for a month long trip pee-covid I forgot to take a litre bottle of spare oil, and after 2000 ++ miles I was getting worrying low. Eventually I had to buy a litre of 10/40 synthetic, as it was all I could find in garages. I poured it all in, which brought me back to the full mark. That was 60,000 miles ago, and the bike is still going strong.
I don't claim to fully get it either.
I was told it wS to do with the chemistry not viscosity per se.
Again, long chain polymers and such... I.e. not just coating the bore with a film better, but the qualities of the lubricant making it that older piston rings, bores etc. would bore glaze and so on.
I've seen it on industrial engines - the small Lister S and T series motors on particular (that might be cos we saw a lot of those in prime power applications).
Run a T series on a "better" oil and it'd do all the things I said and more, and in a relatively short time. About the best way to kill a pretty unkillable motor short of leaving the sump plug out.
Run one on an API CC/CD straight monograde or a low spec 15w/40 mineral oil, and it'd just keep on chugging away.
brightspark
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older engines on motorbikes shouldnt be run on modern syntheyic oil as it has detergent in it and scours the engine and shoves all the crap through your white metal and bronze bearings .my norton runs on 50 grade morris oil , ive fitted a screw on oil filter kit to it so now can be run on synthetic if i want as the old filter was in the sump plug just a gauze and a magnet
barracuda816
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FYI the oil geek on YouTube is imo really very good if you are interested in oil chemistry.
MattF
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If I recall, the reason it could be problematic is that it has some type of detergent effect, so all it would do is merely highlight issues which already existed. Crap/knacked seals, gaskets etc.
Gareth J
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Don't be thinking you can start a "what oil" thread and have it end as quickly as that!
barracuda816
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I have heard that too but never seen in in practice. In any case a pint of synthetic will change nothing by a measurable amount.
brightspark
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it wont harm mixing it and topping up 
It's been years since I've paid that much attention to oils (other than ensuring that I have the correct spec for whatever I'm servicing), but the couple issues I can remember, are oil retention on cylinder walls, and detergents.
Modern oils (Non-synthetic/semi-synthetic/fully synthetic are ultimately marketing BS these days) don't burn as easily and have a higher shear, which means they remain on cylinder walls longer compared with old oils, and don't scrape of as easy. This can reduce oil consumption, however you do risk bore glazing, especially on engines that aren't worked hard from new.
What is of more immediate concern, is putting a detergent oil in something that has always been run on non-detergent oil.
The detergent will gradually dissolve all those years of internal gunk build-up, which if it then exceeds the oils sludge suspension capability, will settle as sludge in the bottom of the sump. Get enough sludge, and it'll block the oil pick up.
Modern oils (Non-synthetic/semi-synthetic/fully synthetic are ultimately marketing BS these days) don't burn as easily and have a higher shear, which means they remain on cylinder walls longer compared with old oils, and don't scrape of as easy. This can reduce oil consumption, however you do risk bore glazing, especially on engines that aren't worked hard from new.
What is of more immediate concern, is putting a detergent oil in something that has always been run on non-detergent oil.
The detergent will gradually dissolve all those years of internal gunk build-up, which if it then exceeds the oils sludge suspension capability, will settle as sludge in the bottom of the sump. Get enough sludge, and it'll block the oil pick up.
