Synthetic oil in old engines

I’ve seen several posts regarding this topic however my engine is much older than what I’ve seen posted I have a 1982 Chevy c10 with a 1979 caddy 425 and I had read many sites saying I need zinc in the oil as if that wasn’t hard enough to find. The previous owner said that he used rotella t desiel oil which didn’t sound like a good idea but he said he did it for the zinc. Any how the problem is I had the oil changed and used Mobil 1 and now I have oil leaking all down the back side of engine. I assume synthetic was bad idea too because it seems to be leaking past the seals looks like at the intake possibly rear main, ect. Now it looks like I have some rebuilding to do and need to find non synthetic oil with zinc, any ideas? Also is there such a thing as too much zinc or zddp if I used an additive.

After you finish with the repairs, I would suggest that you begin using the “Classic” motor oil marketed by the folks who publish Hemmings Motor News. Their formulation contains a high level of both zinc and phosphorus–two elements that are at very low levels in the oils formulated for new cars.

First off you need to find the source of your leak, making sure it’s not just a coincidence. You could have just sprung a leak at a valve cover gasket. Clean it off as best as you can and find out the source of the leak first. Then decide what to do from there.

Modern engine oils aren’t the best choice for your old engine. The things that make them good for a 2011 engine are what make them not so good for a 1979 engine, and vice versa. Your old slipper lifter engine would do better with an older type oil. Depending on the climate where you live I would use Chevron Delo, Shell Rotella, Mobil Delvac and such in that engine. When I still had my 1970 Cadillac 472 that’s what I used.

The hemmings oil looks good except the price… 54.00 just for the oil I’m either out of touch on oil prices or just cheap. Lol
Apparently being synthetic is not the issue since both oils recommended are synt.
Ase master so the desiel oil will not hurt a gas engine? And what viscosity did you use since what I thought was 15 -40 was most common.

I think the diesel oil, Rotella etc., is a good idea.

Yup, go with diesel oil. If you read the back of the bottle you will find that it meets auto-spec oil ratings from only a few years ago, well past your 79 Caddy engine. Syth Rotella T6 is what I use in my older engines.

Use a conventional oil labeled API Service CJ-4 to protect your valve guides and cam.

Red Line, Lucas, Amsoil, Rotella

You can get three gallons delivered from amazon for under 50 bucks.

Costco often has Delo 15w40 on sale

Is extra zinc still needed in '79 engines?

It’s my opinion that the zinc or zddp is needed in any engine with an old fashioned flat-bottomed lifter riding on a camshaft.

I have a daily-driver '94 F150 with the flat-tappet inline 6. I’m using Mobil1 Turbo Diesel Truck 5w40 that I got cheap on closeout. I recommend diesel oil (with the API SL, SM, or SN) for the extra zddp for peace of mind…but a low-revving caddy ought to have fairly soft valve spring pressures, and therefore ought to be fairly tolerant of a modern oil. (It’s more critical for high-revving high-output iron with high spring pressures.) DO NOT USE 0w20 OR 5w20!

Use something with better then a SF rating diesel oil is cheap now(thats what I would use)-Kevin