I attended a session put on by Drive Motor Oil (formerly Joe Gibbs Racing Oil) at the POCI show in Fort Worth TX in 2017. He addressed this specifically.
The problem with using synthetic oil in a vintage motor was the rear main seals (old rope style) will leak. Engines produced after 1997 if I recall are ok, including rebuilt motors as long as they utilized a newer style rear main seal.

When were your 261 & 400 built (or rebuilt is likely more pertinent)?

You will get a million conflicting opinions on this subject!

In my winter car 67 beaumont 250 straight 6 cyl never been apart motor,  I use full synthetic 5w30 all winter long, change it once a year,  been doing it for over 10 years now,  no problems.  BUT i do add 500 ml bottle of gm EOS with zinc.   The zinc would be the only thing missing if you buy a full synthetic off the shelf.  if i was not using in the winter, go with 10 w 30.  I personally dont care if it leaks oil, and it does, but is that a direct result of synthentic?  who knows.

One thing in the back of my mind is that the thin oil could cause spun bearing problems due to the old sloppy clearances of an old motor(they like thick oil to take up the gaps).  but in my case i just want it to turn over and start when it is minus 30 and not plugged in.

every manufaturer will have an opinion, beacause they are trying to sell product, so i really dont pay any attention to that sort of stuff.  im a realist.

-- Edited by beaumontguru on Sunday 2nd of February 2020 02:13:47 PM

I agree with Dave with regards to the use of Zinc additives
When I rebuilt the flat head Six (239 cu in ) for my wife's car, we use original style seals and gaskets. It was a decision we made, not saying it was right, it is what we did simply because we bought a complete Kit. Upon start up of the engine, the manual called for 20W oil , but if the temperatures are expected above 90 degrees F you should use 30 oil. Well, I could not find 20W oil and the car was never to see cold temperatures, so I used Shell 10W-30 ( four liters) " plus 500 ml " of the Lucas Zinc additive. Slightly over the 4 1/4 quarts of oil required by the manual. I never even considered nor would I use Synthetic grade oil in that engine, but absolutely would use Zinc additives. The Engine Oil Pressure runs around 40 PSI @ 50 mph, Since the car was put on the road, we have probably put on 3-400 Klms. So, To answer Long Stroke's question , the car is currently in the hoist for winter storage, there are no oil leaks evident front or rear of the engine, there was a drop hanging in the crank case vent, which I wiped out the other day and a slight dampness around the oil pump base, don't know if that was over spray from my rust check application or not? but I plan to check the bolts in the near future. If in the event of leaks developing in the future, my plan is simple, since it is a summer vehicle, I just going to increae the oil grade viscosity, no worries no stress and certainly no panic LOL
Good luck with your choice
Dale

Zinc is a must for all of our older engines if they have a flat tappet camshaft. An older engine with a roller camshaft would not require Zinc. I am religious about putting Zinc in both my vintage motors because replacing a cam and lifters is a time consuming and expensive proposition....... ask me how i know. I made the mistake of not being religious about the Zinc in my 57 261 and i ended up with some bad lobes which was effecting valve timing. I created a ton of work for myself having to do all the work on my motor over a winter about five years ago. I think i will stick with quality fossil oils. Thanks for the response and cheers.

How about Synthetic ATF, any advantages in the old tranys ?

Years ago I had a truck trade publication newspaper from 1976. In it was an article on synthetic oil. They ran it in a test vehicle, specifically a new but broken-in 1975 Laurentian. They ran it for 50,000 miles then tore it down. Without a doubt the synthetic reduced wear rates, it flowed more consistently, and it clung to parts where conventional oil may sling off. It also works to eliminate sludge (that may be where the leaks start as leaky gaskets were masked by sludge). It will eventually clean the pan considerably if you don't stretch out the oil change intervals.

 

That is interesting about the rope main seals, Clint.

 

Apple to oranges, but in vintage 2-stroke outboards with bronze bushings as opposed to roller bearings, even with synthetic 2-stroke oil one must maintain the correct 16:1 or 24:1 gas/oil ratio in order to have the correct film thickness for the bearing. What Beaumontguru was saying about bearing clearances may mean a higher volume oil pump (not higher pressure) may be needed to float those journals in the bearings. I'm not a builder but those are my impressions. If the bearings are too sloppy then it is rebuild time anyway.

 

On a different note, don't run synthetic gear oil in an older manual trans like a Muncie with yellow metals inside. It will overburden the syncros & wear them out rapidly.

The update now on my 74 400 V-8 using synthetic oil is not a good one. It seemed ok for a while until i noticed oil on the ground. It seems to be leaking at the rear main seal on my 1974 Chev 400 V-8. This car has never and i do mean never leaked since my dad bought the car May 1974. It leaks now and that tells me it is the oil. The next oil change will be non synthetic oil for this baby, i can tell you that. I tried and i lost. Cheers.

-- Edited by long stroke on Friday 19th of June 2020 08:05:10 PM

What are we defining as a vintage engine, what year 1980 and older 1990 and older, plenty of evidence on searching web that "older" trucks benefit from synthetic oil especially in wear and fuel consumption.

seventy2plus2 wrote:

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long stroke wrote:

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The update now on my 74 400 V-8 using synthetic oil is not a good one. It seemed ok for a while until i noticed oil on the ground. It seems to be leaking at the rear main seal on my 1974 Chev 400 V-8. This car has never and i do mean never leaked since my dad bought the car May 1974. It leaks now and that tells me it is the oil. The next oil change will be non synthetic oil for this baby, i can tell you that. I tried and i lost. Cheers.

-- Edited by long stroke on Friday 19th of June 2020 08:05:10 PM

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 Yes, the old rope rear main seals are not compatible with synthetic oil and will leak.

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 Are you meaning his 400 was built with a rope seal? I never say never but so far I've never seen a rope seal on a small or big block Chevy, including back into the early 60's 283's. But I never say never... I would have bet money the 400 doesn't have a rope.

I'm wondering if the oil leak could just be coincidental, and not directly related to the type of oil used.  I've had many old cars with many oil leaks, and honestly have never purchased any of that newfangled (lol) synthetic oil yet (I will if I purchase a new car, but my newest car originally was equipped with conventional oil, so that's what I stick with).  My experience is that they all leak eventually, regardless of oil type used, and an original rear main seal that's 46 years old has performed well past its initial design requirements.

Unless there is some aspect of synthetic oils that accelerates the degradation of old oil seals (and there may be - I'm not a chemist so I can't say), then it may just be time to replace the seals (or live with the leaks).  JMHO.

I use Castrol Syntec 5/30 in my 64, 327 300hp. never a problem. Synthetic in Trans. and rear end.
Brian

In response to the sludge and varnish reference, in the 70s one of the customers that came to our shop did most of his own maintenance such as oil changes and tune ups but we would do more major repairs. His Chevy van had just about 200,000 miles on it and needed a timing chain. When I pulled it apart it was extremely clean inside, no varnish or sludge at all. I asked about what oil he used and he said four litres of 10W30 and half a litre of atf to bring it up to the full mark. He claimed that the high amount of detergent additives in the atf is why the engine was so clean. I tried it in my car and in a few months I started having a rear main leak. I dont know for sure if the atf contributed to the leak but with people having trouble with synthetic oil and leakage it may be due to the different types of additives in synthetic oils.

Paul