I bought my 66 Coronet with the /6 about 18 months ago. Records and visual inspection indicate that the engine has never been rebuilt and the mileage is likely accurate at 91,000. The oil pressure warning light works as it should and has never come on while driving. I just added an AutoMeter electrical oil pressure gauge so I can see at least approximately what's going on. The numbers I see:

Cold Idle: 20
Hot Idle: 10
Cold on highway at 60 mph right after start: 35
Hot on highway at 60 mph: 25 to 30

I am running AMSOIL Z-ROD 10W-30 oil and it has been changed twice, in June 2022 and in January 2023.

Am I OK at this point running as is? Would you recommend changing to 10W-40 oil or other weights?

Thank you for your input.

Jon

The owners manual states that it is normal for the oil pressure lit to flicker dimly when hot and idling in gear. So according to chrysler having only 4 psi at idle in gear on a hot engine is fine. Sounds like you are above the spec.

The general old rule of thumb was to have 10 psi for every 1000 rpms. So cruising down the highway at 2800 rpms would be around 28-30 psi.

Two things I suggest.
1) Service the oil pressure relief valve. Minor job, can't hurt anything, and might raise the oil pressure.
2) Don't waste your money on boutique oils. Not necessary. Plain Jane 10-30 is fine.

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Charrlie_S
65 Valiant 100 2dr post 170 turbo
66 Valiant Signet 170 nitrous
64 Valiant Signet
64 Valiant 4dr 170
64 Valiant 4dr 225

Those numbers are a bit low, but just fine for a stock engine that you are not revving high or abusing. The engine will roll off the power pretty fast above 4000 RPM, so no reason to rev much beyond that. 35 psi is fine for 4000-4500 RPM and probably higher.

I second the notion to not waste money on such oil. 10w30 in normal brands is just fine.

Lou

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Home of Slant6-powered fun machines since 1988

Thank you. I will take a look at servicing the relief valve. So, it's OK to use oil without the extra zinc and phosphorus? I have read some detailed discussions pointed to from this forum that say the need for zinc/phosphorus is overstated.....I wasn't sure.

Thanks. Good to hear.

Yes, I figured my numbers were on the low side but probably OK. So, for best oil flow I should stick with 10W-30? I live in a warm climate, coastal SC, and never run the car below about 40 degrees.

Yep, it will be a good idea to service the oil pressure relief valve to make sure it's doing its job, as described in these two threads: thread 1, thread 2.

Also, you are spending much, much more money on engine oil than is warranted. There is zero real, practical benefit to high-priced boutique brands (Scamsoil, Redline, etc). Even the worst oil available today is massively better in every way than the best oil available when the Slant-6 was in production, or even just 10 or 15 years ago. You don't need to fret about new-fashioned oil in old-fashioned engines or anything like that. Use a good quality filter, and change oil + filter at a reasonable interval (3,000 miles is much too often unless you do nothing but short trips in cold weather—another giant waste of money and oil).

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Thank you, Dan. I do use a quality filter every time - usually Wix or NAPA. I'll trim my spending on oil as well. Is 10W-30 the best formula for my location and usage? I put on about 150 miles a month, temps hardly ever below 50 degrees, and usually long enough trips to warm everything up well. I'll go through those threads on the relief valve.

With an engine in good condition, and those mild temperatures, 10w30 is just fine. Keep eyes open for engine oil sales and rebates, and you'll be able to get a premium name brand oil (Pennzoil, Valvoline, Havoline, Quaker State…) for an easy-on-the-pocketbook price.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Yup on the need for "boutique" oils... Not needed.

I don't get to go to auctions anymore like I used to but a couple of auctions ago that I did go to I somehow ended up buying 3 pallets of oil. Among what was there I had some odd weights I'd never previously had heard of plus lots of SF/SE/CC (spec that was current as of late 80s) in 5-30, 10-30, and 10-40 all in CANS that take the old spout that has migrated to the far back bottom drawer of everybody's toolbox since they started selling oil in bottles. I just used 6 quarts of Pennzoil 5-30 "with Z-7" even (whatever that was) of it last night....
1 whole pallet was type f! ( That stuff is all gone)
I sold off about 2-3/4 pallets worth, and kept about 2 levels/ layers of 1 pallet because that is weights I can use, and I have a few engines around from "back then"
And yes I will use it up.
I figure I'd keep enough of the weights of oil that I could use, seeing as how I'm sure it has plenty of zinc for my flat tappet /6 and 318s....
About 3 months prior I had bought 3 cases (since when is a "case" only 6 qts?) Of Valvoline VR1 10w30 for break in of my fresh /6.... I have only run that engine a couple of hours so far, no road miles yet, next time I start it up will be just long enough to warm it up and get any break in remnants floating so I can get that break in VR1 outta there for a fresh charge... Had I known when I bought the VR1 that I'd have ended up (however temporary as that was, haha) in the oil "business" I wouldn't have bought that VR1....
When my stash of "period correct" oil runs out, I'll probably go back to buying whatever conventional oil is on sale at the local farm store..... I definitely am gonna use up what I have because it's here and once I got rid of what I couldn't use (sold a lot of it to vintage John Deere guys haha) what I kept here wound up being absolutely free.....

And for those wondering "why" I would have bought such a lot?
With ATF +4 being as ridiculous as it is in the stores (I pd $11/qt-$132/ case for some shortly after I bought this auction lot) in the auctioneers website photos posted before the sale, it showed that 2 layers of the on the bottom of one of the pallets that was supposed to be included was ATF+3. Which is closer to ATF+4 than anything else I know that isn't actually ATF+4, that was what I was wanting.
And somehow we had to bid on 3 pallets of oil that was stored off-site. So I couldn't see it day of sale. I bid cheap, nobody bid against me, so there i was
When I went back a couple of days later during "business hours" when someone who had access to the forklift was on duty, I saw then that somehow there was suddenly no ATF+3. I ended up with exactly 1 case (12 bottles) of exactly what I went for... So I sold off all I couldn't possibly use...
(Hopefully) interesting side story there ...

Oof. I realise conversations about this topic are more or less a matter of religious beliefs, but…maybe less with the keeping this myth alive? Engine oil antiwear technology did not start or end with zinc or phosphorus, and even the current engine oil certification tests include flat-tappet engines.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Well hey
Once I sold off 90% of the pile the portion I kept is free.... And using what I kept sure won't hurt anything,, so I'm gonna use it up....