Viscosity Experiment

[Jim Allen]

I'd appreciate a logic test here.

I undertook a little experiment to compare the hot, operating oil pressure on two of my diesel engines when I switched from 15W40 to 10W30. These were a Ford 6.9L V8 diesel with 135K in a pickup and a Ford 152ci 3-cylinder diesel in a tractor with 3K hours.

With the oil at operating temp with fresh 15W40, I recorded the pressure and compared it to the nominal pressure listed by the engine manufacturer.

When I changed to 10W30, I did the same thing.

Ambient temps were approximately the same, though the tests were a year apart, and I took pains to get both engines good and hot. I used the same test gauge on both engines and for both viscosities. Both oils were Rotella-T.

The pressure was about 4-5 psi higher with 15W40 than the 10W30 on the tractor. The pressure was within a psi or two on the truck. Both engines had pressure above the specified pressure listed by the manufacturer with both oils. Here are the numbers, in the "actual" column the 15W40 is list first/then the 10W30. I hope my formatting holds!

Idle Spec RPM Spec Actual Idle Actual RPM
6.9L 10psi 40-70@2000 18/18 50/48

152ci - 40@1000 16/16 48/43

What did this tell me?

Bottom line, I think it told me I was safe for the 10W30 (which is spec'd for both engines by the way in my climate). If pressure is resistance to flow, then the utility tractor, especially, was running too heavy an oil.

What I wonder, though, is if this test would be a valid way to decide if you could drop a grade? I guess this might be more appropriate for the gassers who do that more than diesel guys but whaddya think?

 

[XS650]

Basing oil pressure on observed viscosity is iffy at best. The calibration of oil pumps and factory oil pressure gauges is usually not at all precise.

You are best off using the factory recommended viscosity, then if the pressure appears to be out of spec, find the cause after hooking up an accurate gauge.

 

[Ken2]

Don't go to a lower viscosity grade than the manufacturer recommends (except maybe in winter conditions for which there is no recommendation). (This does not include such things as subbing a 5W-40 syn for a 15w40.)

You have no way of measuring the many thousands of psi of pressure between the bearing surfaces. The oil film must be able to keep the metal surfaces apart, and you won't know if there is metal-to-metal contact until too late. Other things being equal, higher viscosity results in higher oil film thickness and stronger oil film strength.

 

[Jim Allen]

Just so everybody knows,I'm not planning or trying to go to a lower viscosity than 10W30 HDEO. As I said, that viscosity oil is recommended by both aforementioned engine manufacturers in my climate. I just wanted to get off the 15w40 bandwagon for better winter starting without having to send $20 a gallon. Been running the oil for 2 years now.

I was hoping for some scientific commentary on the idea of using hot oil pressure as one measuring tool for a person with a not-new engine who wanted to drop a grade, for whatever reason. I know the oil pressure is one tool in how the engine manufacturers rate the engine for viscosity, though it's much more complicated than that.

 

[HammerDown]

Originally Posted By: Ken2
...The oil film must be able to keep the metal surfaces apart, and you won't know if there is metal-to-metal contact until too late. Other things being equal, higher viscosity results in higher oil film thickness and stronger oil film strength.

I like your way of thinking

 

[BuickGN]

I don't think you can get much from oil pressure. All you get is A- there's not enough pressure so you're doing damage or B-there's enough pressure and you may or may not be doing damage.

 

[tdi-rick]

Jim, when switching back and forth from a 10w30 to a 0W-5 (that isn't a typo) way back in the mists of time in a race engine I observed no changes in hot maximum oil pressure (adjustable pressure relief valve on the pressure pump)
On that particular engine I only ran 30psi @ 7000RPM too.

 

[tdi-rick]

Originally Posted By: Ken2
Other things being equal, higher viscosity results in higher oil film thickness and stronger oil film strength.


Hmm, numbers I've seen often indicate that the lower viscosity oils actually have a higher film strength than a comparable higher viscosity oil, but the film thickness is obviously thinner.

 

[bruce381]

0W-5 (that isn't a typo)
no such animal per API

 

[Gary Allan]

Neither is 0w-10 ..but it doesn't stop a wildcat blender/tribologist from producing it.

Well, we learned (in my special ed class on BITOG) that our visc determines our journal bearing gaps. Assuming some minimum volume of oil through the journal bearing, the thickness will be the same regardless of oil volume ..or oil pressure.

That leads one to conclude that lowering the viscosity will reduce the cushion between your bearings and the journals.

It does not mandate that you'll see increased wear ..at least in the bearings. It merely reduces the margins of the safe operational envelope.

Now I'm off to learn how viscosity has any relationship to wear in the rubbing or meshing surfaces. My observations say "maybe ..but".

 

[firemachine69]

Viscosity is determined by clearances.

 

[tdi-rick]

Originally Posted By: bruce381
0W-5 (that isn't a typo)
no such animal per API


True, but it wasn't an API spec oil either.
Can't remember offhand what the cSt's @ 100*C were, it was over fifteen years ago, but it was extremely thin and poured like water (Neo 0w-5)

 

[tdi-rick]

Originally Posted By: Gary Allan
Neither is 0w-10 ..but it doesn't stop a wildcat blender/tribologist from producing it.

Well, we learned (in my special ed class on BITOG) that our visc determines our journal bearing gaps. Assuming some minimum volume of oil through the journal bearing, the thickness will be the same regardless of oil volume ..or oil pressure.

That leads one to conclude that lowering the viscosity will reduce the cushion between your bearings and the journals.

It does not mandate that you'll see increased wear ..at least in the bearings. It merely reduces the margins of the safe operational envelope.

Now I'm off to learn how viscosity has any relationship to wear in the rubbing or meshing surfaces. My observations say "maybe ..but".


Gary, what I found when using the really light lubes was that I had to tighten up the bearing clearances significantly (IIRC went from 2.2 thou to 1.3-1.5 thou) as I appeared to be getting cavitation erosion of the bearing surface. (Vandervell bearings)
Once I tighten them up, no more problems.
It probably didn't help only running 30psi either. I later changed my bearing make to ACL, who used a harder overlayer, but seemed more dimensionally consistent.

Bore wear was also an issue, (and people using xW-30 oil were also experiencing bore wear issues in that category at that time) so I had the machine shop use a much finer grit hone, started using plasma moly rings (Had been using a black iron top compression ring to be legal. Up to that point they had been 'officially' banned, but I knew of people running them) and finished off with a plateau hone. It seemed to work.

 

[Jim Allen]

So I gather that my idea has little merit as a testing tool. I wonder how to account for the slight changes in pressure I noted? Well, nothing like BITOG to shatter unrealistic daydreams!

In any case I am using the 10W30 in the two vehicles mentioned with no problems. I noted much better cold starting on the Ford tractor, which was my primary worry since it's my road-clearing tractor (I have a half-mile driveway). Two winters ago I had some starting issues with it on a sub-zero day, mostly due to 15W40. I have gone to two year OCIs on both my tractors, so I may be able to justify the cost of a synthetic for that tractor to further aid cold starts. I will crunch some numbers when I get close to the next OC.

tdi-Rock: 0W5... you're a better man than me Gunga-Din. What were you racing? The unasked question... did it give you any advantage on the track... those last few ponies?

 

[tdi-rick]

re the Neo, yes, it was worth it.
I used it in a Formula Ford 1600 as it gave us 4hp and 4lb/ft torque (right through the rev range I might add, better ring seal from the di-ester ? ) over what most everyone else was running.
In a category where you'd sell your granny for 1/10th of a second, it was well worth it. (quite a few of the fellas I raced against are now pro race drivers, with one on F1

)

There's still 5 litres sitting in the shed, never been game to use it in anything since

 

[TallPaul]

Sufficient viscosity in the bearings is critical. I also like a margin of safety over modern 5w20 oils. The critical viscosity for the bearings (IIRC) is the HTHS (High Temp/High Sheer) viscosity. In most cases you actually can drop a viscosity grade below the manufacturers recommended, provided you use a super robust oil like Redline. Redline 5w20 has the HTHS viscosity of a decent 30 weight oil and I have no problem running that in any vehicle I normally would run 10w30 in, provided I maintain adequate oil pressure.

Anyway, fascinating thread, although I only did a cursory read at this late hour.

 

[Steve S]

In my 3 cyl 29 hp John Deere compact tracter the owmers manual oil viscosty chart shows 10w30 below 90*f is OK.Different engine but ????

 

[Taylor]

so basically its fine to run a very light oil as long as the engine is built for it? and if you feel it necessary to run a grade lighter it would be perfectly fine as long as the HTHS viscosity is in line?

 

[TallPaul]

Originally Posted By: Taylor
so basically its fine to run a very light oil as long as the engine is built for it? and if you feel it necessary to run a grade lighter it would be perfectly fine as long as the HTHS viscosity is in line?

I'd say yes and probably, but than I am no expert.

 

[Gary Allan]

Originally Posted By: Taylor
so basically its fine to run a very light oil as long as the engine is built for it? and if you feel it necessary to run a grade lighter it would be perfectly fine as long as the HTHS viscosity is in line?



Sometimes, in terms of bearing wear, it's okay to run a very light oil in an engine that was never built for it. If UOA has any relationship to wear (even one that isn't at all representative of the magnitude of wear), I've got zero and next to none. What I do have is a good refresh rate (less visc change due to less heat soak while passing through the journal). My sump temp is stable (exchanger) ..and I don't race. My idea of high speed is 75mph and my cooling system capacity is never challenged.

YMMV

For your common passenger car use, one or two OCI's aren't going to break any banks. Do a UOA to find out. It should tell you if you're barking up the wrong tree.