Variation of Oil Pressure Between Oil Brands

Moderators: Sluggo, Amskeptic

Post Reply
User avatar
Hippie
IAC Addict!
Location: 41º 35' 27" N, 93º 37' 15" W
Status: Offline

Variation of Oil Pressure Between Oil Brands

Post by Hippie » Mon Aug 30, 2010 4:52 pm

Posting this for general interest to those who may have used two or more different motor oils that claim to be the same or similar viscosity or "thickness" but have seen different oil pressures with them under the same driving conditions.

I am an oil nerd.

The viscosity of motor oil on oil specification sheets (usually on the manufacturer's website or given to you for the asking of their tech department) is given in kinematic viscosity, in units called CentiStokes, or cSt. The higher the cSt, the thicker the oil.
Typical value of an 5W-30 weight oil, just for reference, is around 10-11 cSt at 100 degrees celsius. Around 18 cSt for a 20W-50.

This--the kv-- is the viscosity that is most commonly reported.
There is another vicscosity called the HTHS, or high temperature, high shear viscosity that is less commonly found on a spec sheet.
This is the "thickness" of the oil, again in cSt, at 150 degrees C, while being sheared (stressed) as the oil would be in the bearings.

Since oil thins when being physically pounded by bearings, as well as the higher temperature found in the bearing clearances, the oil temporarily thins to typically 1/3 or less its reported kv.

For reference, a 5W-30 may have an HTHS in the bearings of around 3.0 cSt...about like whole milk. 50 weight, maybe 4.5 cSt.

Since SAE specifies weight based on the 100 C kv, and not HTHS viscosity, there can be a large variation in these figures.

For example, Valvoline Synpower 5W-30 has an HTHS of 2.9 cSt, and Redline 0W-20 has an HTHS of 3.3 cSt.
This means the Redline oil has a much higher film strength under high loads and temperatures of the oil.
There is a correlation between bearing wear and HTHS, but not so much the kv.
The relationship is not linear, however, and anything much above 2.6 does not necessarily mean significantly more long term wear. Much depends on the bearing loading design as well, and modern engines are designed for the thinner oils for minimum wear.

Since the oil pressure is a function of the oil pump pressing against the back pressure of the oil that is in (and being sheared by) the bearings and other moving parts, the oil pressure on your gauge appears to relate to the HTHS of a particular oil, rather than it's stated viscosity at 100C.

Thanks to CATERHAM at bobistheoilguy.com for the following data collected in his own vehicle:

All oil pressure readings were made at 6,500 rpm and oil temp' of 95C:
M1 5W-50 ..... KV100 17.5 cSt...HTHS 4.21 cP...OP 92 ...psi
M1 0W-40 ..... KV100 14.0 cSt...HTHS 3.7 .cP...OP 86 ...psi
RL 10W-30 .... KV100 11.0 cSt...HTHS 3.8 .cP...OP 87 ...psi
RL 5W-30 ..... KV100 10.6 cSt...HTHS 3.8 .cP...OP 87 ...psi
GC 0W-30 ..... KV100 12.2 cSt...HTHS 3.5 .cP...OP 83-84 psi
RL 5W-20 ..... KV100 9.1 cSt...HTHS 3.3 .cP...OP 80 ...psi
M1 5W-30 ..... KV100 11.3 cSt...HTHS 3.09 cP...OP 78 ...psi
PP 5W-30 ..... KV100 10.3 cSt...HTHS 3.1 .cP...OP 78 ...psi
RL blend, 3qts 5W-20 and 1qt 0W-10 race oil *
...............KV100 8.2cSt est HTHS 2.85 cP...OP 74 ...psi
Toyota (Nippon Oil) 0W-20 virgin, less than 30 miles on oil
...............KV100 8.8 cSt...HTHS 2.6 .cP...OP 71 ...psi
Toyota 0W-20 used with 150 miles on oil **
...........est KV100 8.0cSt est HTHS 2.4 .cP...OP 65 ...psi



And here are some graphs I made of the above indicating a 99.4% accuracy in oil pressure predictabilty using HTHS viscosity, vs ~73% for kv alone.
There is some relationship in the latter, but only as far as HTHS tends to go up in oils of thicker weights.

Image

User avatar
RSorak 71Westy
IAC Addict!
Location: Memphis, TN
Contact:
Status: Offline

Post by RSorak 71Westy » Mon Aug 30, 2010 5:59 pm

did you know VW engines bypass the oil COOLER at pressures above about 45 PSI? This is nutz? Why do they do this?....It makes no sense to me.
Take care,
Rick
Stock 1600 w/dual Solex 34's and header. mildly ported heads and EMPI elephant's feet. SVDA W/pertronix. 73 Thing has been sold. BTW I am a pro wrench have been fixing cars for living for over 30 yrs.

User avatar
Hippie
IAC Addict!
Location: 41º 35' 27" N, 93º 37' 15" W
Status: Offline

Post by Hippie » Mon Aug 30, 2010 7:34 pm

I guess they figure if the pressure is that high, and you're using recommended oil grade, then the oil is still cool enough.
I know when that piston opens, it takes a big drop in pressure to close it again.

Air cooled VWs, in my opinion, do well with a high HTHS oil (high film strength) because the main bearing clearances open up with increasing temperatures and that leads to the crankshaft having some room to pound around against the oil film on the bearings.

A too-thick-too-high-HTHS-oil might bypass the cooler for longer, but it will have to heat up and thin out eventually, I suppose.
But if your 20W-50 is running like 60 psi in July in a long highway drive, you might benefit from a lower weight oil.

User avatar
rmannon
Getting Hooked!
Location: Portland-ish Oregon
Status: Offline

Post by rmannon » Mon Aug 30, 2010 9:14 pm

@Hippie

You my man are an oil nerd. I only get like that about linux.

User avatar
Hippie
IAC Addict!
Location: 41º 35' 27" N, 93º 37' 15" W
Status: Offline

Post by Hippie » Tue Aug 31, 2010 4:03 am

I resemble that remark. :blackeye:

User avatar
Amskeptic
IAC "Help Desk"
IAC "Help Desk"
Status: Offline

Post by Amskeptic » Thu Sep 09, 2010 7:46 pm

Hippie wrote:I guess they figure if the pressure is that high, and you're using recommended oil grade, then the oil is still cool enough.
Exactamundo.
:cyclopsani:
BobD - 78 Bus . . . 112,730 miles
Chloe - 70 bus . . . 217,593 miles
Naranja - 77 Westy . . . 142,970 miles
Pluck - 1973 Squareback . . . . . . 55,600 miles
Alexus - 91 Lexus LS400 . . . 96,675 miles

Post Reply