Given that tiredtrucker's 10W30 is clearly not an ILSAC 30 (HTHS ~ 2.9-3.0), and is, in fact the HTHS that a dino SAE 30 generally has in the wild, I think the "thicker is better", and "lucas" comments are just strawmen fired out there for beating down.
Industry standard was SAE30 (HTHS 3.5), and in order to accept multigrades, they accepted 15W40s on actual service performance, discovered high shear viscometry, then started specifying HTHS in J300....as a minimum standard.
(Has had people calling 20s really 30s, 30s, really 40s etc. when they are clearly what the grade was intended).
So diesel engine manufacturers specifying 10W30 - with an HTHS of 3.5 - is not an abandonment of "thick is better", it's an acceptance that lubricant technology has grown to the point that multigrades can now deliver HTHS commensurate with what their grade promises.
If we'd skipped the middle years of underperforming multigrades, we would have been at SAE30 with better cold performance I'm sure (no need for lucas references).
What's interesting with the focus on the J300 viscosity grades is that a simplistic approach could lead one to install an ILSAC 5W30/10W30 into one of these engines.
HTHS of 3 versus 3.5 is significant in terms of engine wear and minimum film thicknesses (that's why it's part of the DD specs, ACEA specs etc.)
But also, in terms of the other parameters in lubricating an engine...topical in a couple of other threads, but piston cooling is one of them....also hinted at in a couple of Honda papers where they express concern that lowering viscosity, losing control of those items that rely on pressure to function.
member CATERHAM has some really good relationships between oil pressure and HTHS...
Quote:
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
It's his engine and operating conditions.
But consider say the Redline 10W30/5W30 with an HTHS of 3.8 and an oil pressure of 87 psi, versus the M1 and PP with an HTHS of 3.1, and an oil pressure of 78psi.
All within the realms of an HDMO versus ILSAC discussion.
In straight piston cooling flow, the flow through a typical squirter the ILSAC 30 will have a 5% lower piston cooling flow at CATERHAM's 95C...for the same J300 kinematic viscosity (XW-30), the lower oil pressure has translated into poorer piston cooling by the tune of 5%.
If there's a ball relief valve in the line to each squirter, it's worse. Throw in a 10psi relief, and it's 77 versus 68psi, or 6% worse piston cooling.
But Used_Oil suggests, a prolonged loaded climb, on a production small(ish) diesel that doesn't have the thermal capacity for it, it's a (possible) death spiral starting off thinner.
If you are on the viscosity controlled,rather than relief valve controlled portion of the pump flow curve, piston cooling in particular drops off with the (square root) of pressure.
Here's one of those 3.0L pistons with what's a fairly serious thrust side issue.
Looking through the missing piece of ring land, you can see the hole that runs from the undercrown, and is supposed to gather the cooling oil, and run it through an internal gallery to cool the crown/ring belt.
Something happened, it go hot and the alloy came apart.
Engine specs 5W30, doesn't mention an ACEA or HTHS.
