Higher oil consumption with synthetic oil

[RDY4WAR]

I was speaking with a local racer this morning about the oil in his street stock car. It's an early 80s Monte Carlo with a tired GM 604 350ci crate engine. He said he's always run Valvoline VR1 20w-50 conventional and changed it halfway through the season. He's said it would consume about half a quart per 100 laps. Earlier this year, he switched to Mobil 1 15w-50, liked how it ran, but said the consumption doubled to a quart every 100 laps. He went back to VR1 conventional and the consumption went back down.

What would be the reason for higher oil consumption with the synthetic oil?

 

[PimTac]

Those old Monte Carlos would blow oil like no tomorrow. If he ran it on conventional for a long time then switched to Synthetic, the chances of oil loss and consumption go up. It happens a lot especially with old cars like that.

 

[Gokhan]

Viscosity makes a huge difference in oil consumption if you have worn valve-stem oil seals. Before I replaced them, my engine would consume 1 quart in 1,000 miles with 15W-40 but 2.5 quarts in 1,000 miles with 5W-30. After I replaced them, the consumption went to nearly zero even with 0W-20.

I'm sure VR1 20W-50 (no HTHSV data provided) is quite a bit thicker than M1 15W-50 (HTHSV = 4.5 cP before permanent shear), especially after some of the heavy dose of the viscosity-index improver (VII) in M1 15W-50 shears away, and that's why he's seeing a difference.

Conventional vs. synthetic makes no difference in the amount of oil consumption, other than a relatively small NOACK effect if the NOACKs are different, but the smell of the burned oil will be different.

 

[Linctex]

Originally Posted by RDY4WAR
.....with a tired GM 604 350ci crate engine.


Time for a fresh engine?

 

[JAG]

I still have not figured it out. Sometimes changing chemistry and increasing viscosity (HTHS viscosity) can cause drastically increased oil consumption. Happened in my oil burner from day 1 Subaru Impreza I had. It burned almost half a quart per 1000 miles with Valvoline synthetic 10W-30 and it almost doubled with Red Line 10W-30. It also was not due to volatility because Red Line had a much lower volatility. I thought it may be a fluke but it was repeatable by using Valvoline, going to Red Line, going back to Valvoline, back to Red Line, and back to Valvoline. I also tried Pennzoil SAE 30 and it consumed at the same rate as Valvoline.

Something odd must occur and I suspect it has to due with piston/ring deposits changing when using some oils AND/OR there is a difference in some oils' attraction to surfaces such that creeping past pistons/rings/valve seals affects how much oil is transported into the combustion chambers. I've seen it in my high temperature tests how some oils will spread out on a steel surface much more than others.

 

[demarpaint]

Originally Posted by JAG
I still have not figured it out. Sometimes changing chemistry and increasing viscosity (HTHS viscosity) can cause drastically increased oil consumption. Happened in my oil burner from day 1 Subaru Impreza I had. It burned almost half a quart per 1000 miles with Valvoline synthetic 10W-30 and it almost doubled with Red Line 10W-30. It also was not due to volatility because Red Line had a much lower volatility. I thought it may be temporary but it was repeatable by using Valvoline, going to Red Line, going back to Valvoline, back to Red Line, and back to Valvoline.

Something odd must occur and I suspect it has to due with piston/ring deposits changing when using some oils AND/OR there is a difference in some oils' attraction to surfaces such that creeping past pistons/rings/valve seals affects how much oil is transported into the combustion chambers. I've seen it in my high temperature tests how some oils will spread out on a steel surface much more than others.


Very interesting, so much for the NOACK?

 

[CR94]

Originally Posted by JAG
I still have not figured it out. Sometimes changing chemistry and increasing viscosity (HTHS viscosity) can cause drastically increased oil consumption. ...
Well, there's the hypothesis that high viscosity oil combined with low-tension rings can lead to excessively thick oil film remaining on cylinder walls when the piston descends. (I've read that, but don't know how often it actually occurs, if ever.)

 

[Patman]

Originally Posted by PimTac
Those old Monte Carlos would blow oil like no tomorrow. If he ran it on conventional for a long time then switched to Synthetic, the chances of oil loss and consumption go up. It happens a lot especially with old cars like that.


It wasn't a Monte Carlo engine in there though, it was a GM crate motor.

 

[JAG]

Yeah, I've also heard of the hypothesis about viscous oils not being sufficiently scraped off of cylinder walls. Seems plausible.

I forgot another personal anecdote. It happened within the last year with my 4-cylinder Ford Ranger that I bought used with 72k miles or so. It had no detectable oil consumption with the oil it came with or two fills of M1 0W-20 EP, which came right after. Then it consumed a noticeable amount of a 45/55% mix of Amsoil SS 5W-20 and 10W-30 as well as 0.36 qts of Hyper-Lube Zinc Replacement polymer esters. That was the fill that got dark very quickly, so perhaps something related to piston/ring deposits was changing. Or maybe it was due to something else. Again, it was not due to volatility. I'm going to run that same mix again, so I'll see if the pattern repeats. I don't think it was due to viscosity because M1 0W-20 EP was the lowest, then the above mix, and the highest viscosity was the Valvoline Premium Blue Restore 10W-30, which had no detectable oil consumption.

 

[RDY4WAR]

He has a catch can on the PCV system which he said had no extra oil in it with Mobil 1 vs VR1.

The 604 engines for the class are sealed engines with OEM 5/64" x 5/64" x 3/16" rings and OEM tension. I think he said it's 7 years old which would mean it has around 8000-8500 laps on it at a 3/8-mile track. He switched to synthetic because of the consumption starting and lap times dropping. He was hoping it would give him another couple years with the engine before rebuilding it.

 

[Gokhan]

Again, it's not the chemistry (synthetic vs. conventional) but viscosity and NOACK. If the oil consumption happens through the rings, higher viscosity probably increases the consumption. If the oil consumption happens through the valve-stem oil seals, lower viscosity increases the consumption.

"It must be recognized that viscosity governs in large part complex oil transport mechanisms to the upper regions of the piston - ring - liner system and therefore affects oil consumption. The low-shear viscosity affects different oil-transport mechanisms in different regions on the piston surface such as inertia forces due to piston movement, gas-flow dragging, and the interactions between the rings and grooves. The HTHS (High-Temperature, High-Shear) viscosity is believed to influence ring and piston liner lubrication and thus governs the oil transport on the liner."

In fact, this paper looked at the consumption of synthetic vs. conventional oil, and they concluded that the only factors that contribute to the consumption are low-shear viscosity (KV @ operating temperature), HTHSV, and NOACK. Conventional vs. synthetic otherwise makes no difference.

The contribution of different oil-consumption sources (PDF link)*
*The contribution of different oil-consumption sources to total oil consumption in a spark-ignition engine
Ertan Yilmaz, Tian Tian, Victor W. Wong, and John B. Heywood
Sloan Automotive Laboratory, Massachusetts Institute of Technology

 

[RDY4WAR]

Originally Posted by Gokhan
Again, it's not the chemistry (synthetic vs. conventional) but viscosity and NOACK. If the oil consumption happens through the rings, higher viscosity probably increases the consumption. If the oil consumption happens through the valve-stem oil seals, lower viscosity increases the consumption.


I'd heard of this before, but only in relation to older engines using low tension rings and deeper valleys in the cylinder wall finish. Is this still true for a more modern engine with a finer cylinder hone, tighter piston to wall clearance, and thinner rings with high tension?

 

[Oildudeny]

20w50 available in full synthetic, why did he opt for 15w50?

 

[RDY4WAR]

Originally Posted by Oildudeny
20w50 available in full synthetic, why did he opt for 15w50?


Probably because he could get it off the shelf at Walmart like VR1.

Something I though about is it could just be a compatibility problem. The residual VR1 left behind in the engine didn't like the synthetic oil added to it. Makes me wonder if after 2-3 changes of Mobil 1 if the consumption wouldn't have slowed.

 

[FCD]

I have used VR1 20W-50 Dino on an off on one of my cars for the past 2-3 years.

The HTHS vis. is a stout 5.0.

It does noticeably increase the hot idle viscosity specially in summer, even compared to other 20W-50 oils ( engine has around 150k miles and factory spec was API SE 20W-50 )

I changed the valve stem seals 2 years ago now and oil consumption after changing them gradually came down to nearly zero after a few months.

 

[OVERKILL]

Originally Posted by RDY4WAR
I think he said it's 7 years old which would mean it has around 8000-8500 laps on it at a 3/8-mile track. He switched to synthetic because of the consumption starting and lap times dropping. He was hoping it would give him another couple years with the engine before rebuilding it.


Switching to synthetic after the horse was already out of the gate so to speak. If the engine had started to use oil and lose power, the damage is already done, nothing that can be poured in the fill cap is going to fix that.

 

[talest]

Originally Posted by Linctex
Originally Posted by RDY4WAR
.....with a tired GM 604 350ci crate engine.


Time for a fresh engine?


Or a rebuild of said engine. It's losing oil from somewhere more then likely some rings

 

[dave1251]

Originally Posted by RDY4WAR
Originally Posted by Gokhan
Again, it's not the chemistry (synthetic vs. conventional) but viscosity and NOACK. If the oil consumption happens through the rings, higher viscosity probably increases the consumption. If the oil consumption happens through the valve-stem oil seals, lower viscosity increases the consumption.


I'd heard of this before, but only in relation to older engines using low tension rings and deeper valleys in the cylinder wall finish. Is this still true for a more modern engine with a finer cylinder hone, tighter piston to wall clearance, and thinner rings with high tension?



VR1 has is quite a bit thicker than M-1. If the thicker theory were true in this case VR-1 would be consumed more.

 

[Gokhan]

Originally Posted by dave1251
Originally Posted by RDY4WAR
Originally Posted by Gokhan
Again, it's not the chemistry (synthetic vs. conventional) but viscosity and NOACK. If the oil consumption happens through the rings, higher viscosity probably increases the consumption. If the oil consumption happens through the valve-stem oil seals, lower viscosity increases the consumption.

I'd heard of this before, but only in relation to older engines using low tension rings and deeper valleys in the cylinder wall finish. Is this still true for a more modern engine with a finer cylinder hone, tighter piston to wall clearance, and thinner rings with high tension?

VR1 has is quite a bit thicker than M-1. If the thicker theory were true in this case VR-1 would be consumed more.

My points:

• Synthetic vs. conventional has no direct effect on oil consumption. If the oil find its way into the cylinders, it will be consumed. If it's synthetic or conventional, it won't magically find its way out of the cylinders.
• Oil consumption is driven by three variables: KV, HTHSV, and NOACK. In a healthy engine, NOACK (oil evaporation at the rings) is about 1/3 of the oil consumption. In a unhealthy engine, KV and HTHSV (oil transport through valves and rings) far exceed the NOACK consumption. Also, PCV oil consumption (droplets of oil mist through the intake valves) is independent of NOACK.
• If valve-stem oil seals are worn, thicker oil will decrease the consumption dramatically, up to several fold.
• If the rings or linings are worn, I don't have a clear answer but thicker oil doesn't necessarily help and it may even hurt. The paper I linked earlier in this thread didn't study thin vs. thick but this excerpt reads that the effect is large and complicated: "It must be recognized that viscosity governs in large part complex oil transport mechanisms to the upper regions of the piston - ring - liner system and therefore affects oil consumption. The low-shear viscosity affects different oil-transport mechanisms in different regions on the piston surface such as inertia forces due to piston movement, gas-flow dragging, and the interactions between the rings and grooves. The HTHS (High-Temperature, High-Shear) viscosity is believed to influence ring and piston liner lubrication and thus governs the oil transport on the liner." In any case, it's a very good paper and read it if you're interested in understanding oil consumption.
• It's absolutely true that once the valve-stem oil seals or rings are worn, the damage is permanent and a better oil won't reverse it.
• The racer RDY4WAR mentioned in the original post claimed something but we don't really know if it's true or false -- people often judge things incorrectly, especially when only a single experiment is performed. Repeatability is the key before such claims can be verified.
• If an oil performs better over another, certainly use the better one. Sometimes experimentation helps to choose a better oil.

 

[OilUzer]

Gokhan nice to see you back!

There was this theory about old(er) cars that synthetic oil (due to its superior cleaning agents) will cleanup the sludge blocking the tiny holes ... and the engine will start leaking or losing oil ... However if that was the case, going back to conventional shouldn't solve the problem (oil consumption) since the sludge cannot buildup quickly enough to fill up the tiny holes again!
It's one of those theories that never made sense to me unless there is more to it!