The Stovebolt Site Forums The Shop Area Engine Shop Engine Oil Weight - Why Do Guys Run Heavier Weights?

Throwing this on here as I am faced with lower than ideal oil pressure (30lb at start maybe 8-10lb after warm up 1955 3800 235CI bone stock). I run 10w-30 rotella, run a remote oil filter. Planning to switch to Lucas hot rod and classic at 10w30.

First off - how accurate are the dash gauges?

To the root of my question, what is the benefit of running a higher weight oil? Does it actually flow better throughout motor and lubricate or are you just artificially boosting your numbers to no real benefit.

What do you guys think?

I know it is comparing apples to kumquats, but the factory specs for the GMC six cylinder engines call for a minimum of:

40 psi oil pressure at 2750 rpm on a new or newly rebuilt engine
25 psi oil pressure at 2750 rpm on a used engine

5 psi oil pressure at idle rpm on a new or newly rebuilt engine
2 psi oil pressure at idle rpm on a used engine

Oil recommended for use between 32 and 110 degrees F was SAE 20

I measured the oil pressure relief valve at 58 psi.

That’s surprising numbers to say the least!

Again, apples to oranges here. Non-rebuilt 283 V8, I run 5W-30 synthetic 'cuz I stock a lot of it for other purposes. No oil related issues and it runs 30 psi hot wound out.

I should have said "at operating temperature" in my post above.

My '52 228 GMC engine still has its original pistons. I don't know if the crank and rod bearings have been changed. I've driven it over 40,000 miles since I bought it in 2013, but the instruments had been changed out for newer electronic gauges just before I bought it so I have no idea what the total mileage is.

When idling hot with the A/C on at 400 RPM oil pressure is 8 PSI. If I turn the A/C off the idle goes up to 425 RPM. That 25 RPM gain increases the oil pressure to 9.5 PSI. These measurements were taken 8 years ago with a 4" diameter 270 degree sweep oil pressure gauge that was supposedly correct +- 1psi. Oil is 15w-40 Rotella.

I use 20w-50 in an unrestored 216.

10w-30 seems thin to me for an engine that old that hasn't been restored. I'd at least bump up to 15w-40.

The 216s are spec'd at 15psi at operating temps. I think the 235s are supposed to run at 30psi at operating temperature.

As HRL said on here once, an engine can run at zero psi as long as the oil being applied to the wear surfaces is replaced to the pump at the same rate.

Does it actually flow better throughout motor
No, slower

I've said it more than once- - - -any oil pressure at all indicates the pump is providing more oil than the engine needs- - - - -AS LONG AS THAT OIL IS GETTING TO THE LAST PLACE IN THE ENGINE THAT NEEDS TO BE PRESSURE LUBRICATED. After that, splash takes over and lubes everything else. Most engines with idiot lights turn the light on at 7 PSI, so no one realizes just how low the pressure can go and still keep all the moving parts slippery. Oil pressure above 30 or so at cruise speed is just wasting energy that could be better used for moving the vehicle down the road.
Jerry

Wow, are you sure that isn't a misprint? Most always it was SAE30 in the day?

When it comes to oil and viscosity, I try to relate to the important and costly engines. Most all large piston aircraft engines in the day used SAE 60.
Most all large marine as well as locomotive engines use SAE 40, and even Detroit diesel 2 strokes only recommended SAE 40. Reason viscosity takes the weight, maintains oil film.
None of the posted engines ever use multi viscosity oils.

Not a misprint. Here's the page from the 48-51 Truck Service Manual. LINK

1965 Pontiac's and most likely all other years, had an low oil light on the dash if they didn't have oil gauges. The sender would turn on the light around 3 or 4 psi of oil pressure, and it didn't matter which engine it was. Heavy oil just wears out distributor gears, put more strain on the timing chains or gear train, and in my opinion doesn't lubricate as good, its to heavy to splash around and get where it needs to be. Thicker oil can bring oil pressure up on worn out engines, but if it doesn't reach the top of the engine while cold, you wear the valve train waiting for it to warm up.

My 250 has 5-20 in it, runs 10 psi @ idle hot. My oil gauge is a stock 30 lb gauge, I don't know what the pressure is after 30 or care to know. Anything above not rattling the valve train is good enough!

I just rebuilt and fired up a 468 CI Pontiac, everything brand new in it. I used 30w Penn Grade break-in oil. I primed the engine with a Dewalt 1/2" high torque corded drill, the pressure gauge showed 60psi. It was all I could do to hold the drill, if you think heavy oil doesn't rob horse power, just prime one someday and see just what it takes to turn the oil pump.

Even a 20w50 should lubricate the top end as well on a cold start, as the Sae 20 that the shop manual mentions.

The heavier multi-grade oils (15w40, 20w50) do offer some piece of mind in that they are “allowed to” (by modern standards) have a higher level of ZDDP than those in the 30 range. You are more likely to get a decent ZDDP level with those heavier grades, but unless the manufacturer will share the level or you submit a sample for analysis, you can’t just assume as much. Some diesel oils have had their zinc levels greatly reduced and the high calcium detergent in them further reduced the effectiveness of whatever is there.

For some heavy “reading” on the subject, go check out some of of Lake Speed Jr’s videos under the “SpeeDiagnostix” channel. You can geek out for a few hours if you like.

My oversimplified take on all this: Most of the engines represented on this site are some degree of worn out. 15w40/ 20w50 probably ain’t a bad idea. Excess viscosity/pressure DOES rob horsepower, but it’s not like many of us are trying to shave off tenths at the drag strip.

I am just struggling to understand if the heavier weight oil actually does anything.

If I’m trying to think about it logically I can only think the thicker oil is harder to push around the motor and probably more strain on the motor than benefit.

Sure the gauge goes up, but I can’t imagine your actually lubricating anything any better than the spec’d oil.

A heavier weight oil would remain in the bearing surfaces longer (or have a thicker layer stay behind), which helps lubricate things on startup, when a large part of the wear happens.

I have never ever seen a worn out distributor gear from running straight SAE 30, that is all my dad ran in the day. And again those gears have much better protection from the thicker oil, since viscosity is what gives the lubricant its load carrying capabilities. And as mentioned above it will cling better for the cold starts so no worries about reaching the top end. Top end? Just look at some very old and large marine engines the top end was lubed by way of oil cups and a man with an oil can.
Besides even a very viscous oil like SAE 60 will be almost like water when at operating temperature, well yeah a bit thicker but.
Oh I worry more about the roll pin that some like ford uses for locating the distributor gear than the gear itself.

I had a 57 Pontiac with Rochester FI. The injection pump was driven off the distributor. They used to shear off a drive pin in the shaft that went to the pump but I don't recall seeing the distributor pin sheer.

I've been using 20W-50 in my '56 Cadillac for over 20 years and this car takes regular trips to Florida and back at 80mph with no adverse effects.

I use the heavier oil because of its the sticking power on the wear surfaces and because it fills up the increased clearances in old engines. It also slows or even stops some oil leaks.

I use to run 20w50 in old high mileage engines. Then I backed down to 10w40 .

Had a smokey old 307 in a 69 C10 that I poured anything in. Never blowed up, just used alot of oil. Gave it away when I found that 402.

I use 10w30 + STP in old motors now and it does fine.

Newer stuff gets what the book says.

Wow that is a rare one. I fooled with those a lot when I was a youngster though on Chevrolet. Do you have any pictures of it?

It's complicated and a lot of folks don't have a complete understanding what exactly those SAE number really mean.

Encoded in those numbers are variables like Low Temperature Cranking Viscosity, Low Temperature Pumping Viscosity, Low Shear Rate Kinematic Viscosity (at 100 C max.), and High Shear Rate Viscosity (at 150 C min.)
On top of that, you have different shear rates at different parts of the engine.

Also the viscosity is dictated by your bearing clearances, in relation to you oil temperature.

Let's say you have a clearance of .0034-.0039 on your mains, under 160F oil temperature you are fine with a 10W-40.
If you temp is over 220F, you need a 20W-60.
Also the tighter your clearance , the thinner your oil needs to be.

Personally I stick with the original manufacturer viscosity, especially since I don't drive in the winter, when there is tons of salt on the roads.
So my cold start temperatures are usually just around 60F and up anyway.

My Dad used to say that he ran a little thicker oil because it circulated a little more slowly so it was able to take more heat out of the areas it was lubricating, then stay in the pan a little longer to help dissipate it. I don't know how true that is, but every engine he ever owned saw the high side of 300K miles. Unless it was already worn out when he got it, that is.

It's interesting to note that the big inch Cadillac V8s (368-425-472-500) only had about 40psi from the factory. Those engines have a lot of bearing surface area-more than a big block Chevy or even a Pontiac, so the lower speed of oil going through everything would tend to bear his theory out. The only Cadillac V8 I ever knew that had a lubrication related failure, was the one with the aftermarket pump set to 60psi. It froze up solid on me, on the way to St. Louis. That was an expensive tow.

So after that, I went back to the factory aluminum housing oil pump that puts out 42psi on a cold day, 38 on a really hot day, and have had many trouble free miles with said Cadillac V8 pushing a 15,000lb converted school bus around.

What weight oil do you use in the bus engine?

Delo 15w40 because it's got higher zinc content which seems pretty important to old flat tappet engines, at least the Cadillacs. In a Cadillac car you can run 10w30 just fine, but it's seeing half the RPMs in front of TH400 in a Coupe that it is in front of an SM420 and 'slow and slower' two speed rear axle. I tried the thinner weight, modern synthetic oil in the previous engine that failed. So, I went back to the heavy duty diesel engine oil and that's working so I'll keep running it.

Gentlemen, let's bring this convo back to Stovebolts and *their* engines, please.


John

FWIW -- I'm running 15W40 (diesel oil) in my 261 because my engine's crankshaft was turned by a race engine rebuilder, thus it has greater tolerances in the mains.

Plus, that's what Jerry said to do and I follow his advice