I ran across this recently and thought it was worth posting. It's not new info for many people, especially those who build a lot of engines. However, there are fairly regular questions posted about "What oil weight should I use?", and I'm hoping the info below on viscosity vs. bearing clearances can help people figure this out more logically.

Also, any "real world" experiences that people can post here which seem to fall outside of these guidelines (successes OR failures) would be of interest, too.

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Years ago, NASCAR engine builders discovered that if they ran crankshafts with smaller diameter journals, they could pickup some extra horsepower.

Reducing the circumference of the bearings reduced friction. A crankshaft with smaller journals was also lighter and revved faster.

The smaller journals also reduced the strength of the crank, so there was a limit as to how far they could go without dangerously weakening the crank.

Also, reducing the diameter of the journals increased the loading on the bearings. The larger the bearing, the broader the area over which the load is distributed.

The smaller the bearing, the more concentrated the load is over a given portion of the bearing. Tightening up the bearing clearances and using lower viscosity oil helps distribute the load over a broader area of the bearing.

Regardless what type of bearings are used in an engine, clearances have to be precise and correct for the oil viscosity that will be used to lubricate the motor: Thin, low-viscosity motor oils require tighter bearing clearances than heavier-viscosity racing oils such as 20W-50, straight 40 or 50 weight oils.

Some recommended rod bearing oil clearances for various oil viscosities:

.0015 to .0018 for 0W20 or 5W-20

.002 to .0024 for 5W-30 or 10W30

.0025 to .0029 for 10W-40 or 15W40

.0030 to .004 for 15W-50 or 20W-50


For main bearings:

.0015 to .002 for 0W20 or 5W20

.002 to .0025 for 5W-30 or 10W30

.0026 to .003 for 10W-40 or 15W40

.0031 to .0041 for 15W-50 or 20W-50

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I like to use the old rule of .0010 clearances for each one inch of crankshaft journal diameter regardless of oil weight used on my street motors and drag race motors
Oil does two things in the motor, it lubricates and it cools. Thinner oil moves quicker when cold and on start up
I now use 5w20WT on most of my builds, street or race
I saw 8 HP loss due to colder oil temps. on the first motor I dyno tested, it gained 8Hp going from 130 F oil temps to 170F oil temps with straight 30 WT oil I tried 10W30WT oil on another motor later on a different dyno , same brand, and saw 3 HP going from 130F to 165 F. I saw no change using 5W20Wt and 5W30WT oils from 80 F to 220 F oil temps.
EDITED: I don't build or race high RPM (above 8000 RPM) motors like NASCAR or any other extended high RPM motors like them so I have no real or valid input based on my experiences on those type motors!
I would love to work with a team that does have a successful motor program to help me learn more about that end of racing
Thanks Brad for posting this

The winter grade number is only meaningful if you are running below 32* F or colder. A 5w30 or a 0w30 means nothing unless it's freezing or colder. It's still a 30 grade oil, which as we know, viscosity grades are a mile wide.

Yep. I have this chart in hard copy at home to compare different brands of "same weight" oils:



I've always thought the ranges across grades were highly inconsistent and came up w/ my own scale for comparison that IMO better equalizes the ranges across the viscosities. But it's just something I did 'cuz I wanted to, not that it's any true scientific tool for comparison.

That's a damn cool chart.

Notice how close a 30 grade engine oil is to an 80 or so gear oil?

I need to find myself a hard copy of that chart.

One more that has the #s for each viscosity range, not just a chart:


That one is available here: http://jdub.supramania.com/Oil%20Pics/SAE%20Viscosity.JPG

For most musclecars and bracket guys it is hard to go wrong with 30w oil. If you park the car outside and drive it when the weather is cold then a 5w-30 isn't a bad idea but if the car is parked inside then a single weight 30w probably works just fine.

I run 5w-30 synthetic in my dyno mules. The lower cold viscosity makes it a little easier to prime them and stuff like that. The straight wt oil can get a little thick on a cold morning.

I believe it's imposible to go to thin on the initial number. 00x always. We're talking cold start up. Oil is ALWAYS to thick. As far as vicosity vs temop. Drag motors don't gain a lot of oil temp. Mine seldom exceeds 160 with a diaper. 0W30
for me. STANDARD 440 main@.004", Chevy rod .003", idle 60psi, 75@7000, still to high. I've cut the relief spring 1/2 coil twice, adjustibe relief screw backed all the way out. I swapped the last rod bearings at 300 passes, looked great.
Doug

What's really cool to me is PS engines run at 10,000 rpm, 1500hp with oil as light as kerosene, and have less oil pressure than my hot idle pressure. The bearings look perfect too. Just Amazing to me

From the dual-purpose car perspective, I know my oil gets plenty of heat during street use, including the trips to the track. For clearances in the .0025 to .0030 range, I'm thinking anything lighter than an Xw40 is going to thin out too much.

Also, comparing the HTHS (High Temperature High Shear) ratings on pretty much any brand's oils shows significantly lower HTHS #s with every drop in viscosity. Maybe this is something that is more of an issue as the oil breaks down over its service life (i.e. not an issue for light viscosity race oils that are changed on a frequent basis).

I think Driven has a chart on its website that shows how different grades of oils thin out across a broad range of operating temperatures which could be of interest, especially when the subject of drag engines that are run at low at low temps is raised.

On the last motor I had on the dyno,(426 wedge) using 15/40 oil I saw a 20hp difference between cold and hot oil. Dave

As has been didcussed before, You can only tighten the bearing clearance if your parts dont flex. For most of us here that means run it loose. For what it's worth I run synthetic, 100 passes between oil changes.
Doug

Numbers get skewed when going from petroleum to synthetic?

The first motor, 1962 pump gas 426 M.W., I dyno tuned lost 30 HP going from Valvoline 30 WT detergent oil to Mobil 1 10x30 WT
It was slightly seeping oil through the intake port gaskets on #2,#5 and #7, those spark plugs where slightly shiny before the oil change. After the first pull with the Mobil 1 they where all (8) oiling bad Synthetics lubes will leak past gaskets that regular lubes won't
I've heard that most motors that are sealed up good will gain some HP and torque changing to a good brand of synthetics lubricants I haven't done any other test on them since back then in 1991, once bit, twice shy

They keep the engines very cold, the block and crank are very rigid and they have control over oil flow like nobody's business. Plus a bunch of other stuff we don't know.

I do know at one time Greg Anderson was running a connecting rod that didn't need a bearing.


For the rest of us mortals, a good quality synthetic oil in a 40 grade is what you need. I cringe when I see idle oil pressure under 30-35 hot. Just no need for it.

I wouldn't expect them to, since measuring viscosity wouldn't take that into account. I know some folks say that switching to synthetic allows you to run a thinner oil... but I don't look at it that way.

I'm in the same camp as David Reher with regards to viscosity in bracket/street strip motors.
10/30 in the spring and fall, 15/40-20/50 in the hot summer months.

Fwiw we've tried all kinds of oils in my friends stocker motor.
The most difference we've seen between 15/40 petro oil and thin runny race oil was 4hp.

I was going to say something similar for drag race applications. I typically saw between 130-160F oil temp. And some days it took serious engine warm up to get the oil up to 130F.
At 140F, I am thinking a 10W-30 would be about 90 SUS.
At 200F, I am thinking a 20W-50 would be about 100 SUS.
IMO, Pretty close at associated operating temperatures.
And as previously posted, specific brands can vary from each other. I have been using 10W-30 in my drag cars for years. But in my street/strip car, I use 20W-50.

I agree with what BradH said: It wouldn't surprise me if after driving 30 minutes with water temp around 180F, the oil would be around 200F.

I never saw the oil temps exceed the water temps in my old street Duster, same thing on the tranny temps. It had the stock OEM 1970/1971 6 quart street hemi Mopar brand oil pan on it with no oil cooler
That was with 5w20WT Valvoline petroleum oil and loose (.003+ rods and.004+ main) bearing clearances