Mixing oil viscosities (again)

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Staff member
May 27, 2002
Guelph, Ontario
Here is a post from Ls1.com from a friend with a Corvette who solved his oil consumption
problems by mixing 4 quarts of Mobil 1 10w30 with 2.5 quarts of 15w50:


As some of you know, between the '99 SS and my 'Vette, I've tried just about every fix out there for the oil
consumption problem including PCV valve mods, using the "new, improved" PCV valve, changing the PCV valve every
few thousand miles, using a Metco oil breather, etc, etc, all with limited success.
Well, a few weeks ago I tried a new angle. I drained my oil and added a mixture of 4 quarts of Mobil 1 10W30 to 2.5
quarts of Mobil 1 15W50 (the 'Vette holds 6.5 quarts total). This past weekend we went on an overnight trip to the
Finger Lakes region of NY and put about 500 miles on the car. The temperatures were in the 90s all weekend long
and on the drive home I pushed the car pretty hard with quite a few WOT passes on two lane highways. Always in
the past this set of circumstances (hot weather and hard driving) would have guaranteed a loss of a quart or two
of oil. I'm happy to report that when I checked my oil level yesterday I hadn't lost a drop! Yeah, baby!

My conclusion? As others (including Patman) have already stated, the 5W30 motor oil that is recommended for our
cars is just too ****ed thin for a performance car/engine especially when driven like one.

For those of you that are concerned about mixing oils/viscosities, I recently spoke to a friend of mine who is a
former research engineer for Mobil and the current technical support provider for Penske Racing's NASCAR and CART
programs. I told him exactly what I did and asked him if it would cause any problems.

This was his response:


Other than a slight loss in fuel efficiency or power, there is no problem in mixing the 10W-30 with 15W-50. You
probably end up with an oil around 10W-40. However, since GM does not reccomend the use of 15W-50, it could
cause some warranty concerns.

My response to that was, "You mean these cars come with a warranty?!"

Anyway, for those plagued with oil consumption problems, you may want to give this remedy a try.

Good luck!

NJ State Trooper #3936 (retired)
Life Member of: Veterans of Foreign Wars & Vietnam Veterans of America
2000 Millennium Yellow Corvette Convertible, black leather, A4

Patman-Trooper seems to have hit on something. I see you're at 999 posts, just thought I'd help you make 1000!
Have a good one!
So why not just try a 10w40??

or a 15w40?

Mobil make the 10w40 for the motorcycles,
I'm sure the corvette won't realize that it has 4 extra cylinders and 2 more wheels
This gets to one of the reasons that here where maintenance is lousy, parts are usually chineese or other low quality, and I believe it is better to stretch as far as possible without letting anyone take apart your engine, Once an engine starts consuming oil I switch them up a grade. I figure it has enough miles on it to have plenty of clearance, and can hold on better. Volvo trucks make up a large portion of my motor oil sales, and i can often reduce up to 12 qts of added oil between changes by upping visc. from 15w40 (recommended) to 20w50. I wrote to a Volvo engineer in Sweeden and he wrote back that you shouldn't ever use anything but 15w40 no matter what. That once it starts consuming oil you should rebuild it. Most local truckers instead of rebuilding will switch to 50 wt, or one I found was using 85w140 in his engine. I prefer the 20w50 route, give them another 100,000 miles or more before tearing down the engine, and in 6 years I have hundreds of happy truckers with this change. (Try starting an engine at -10F with 50wt or 85w140!)
It would make sense for him to go to a 10w40 or a 15w40, but he (like many people) wants
to stick with Mobil 1, since it's easy for him to find. So this mixing method gives him the viscosity
he wants, but he can easily find the oil. I have suggested other brands, but a lot of people
are brand loyal to Mobil 1.

My 1000th post! It's been a ton of fun so far, I'm looking forward to thousands more posts!

You guys behave in here today, I'm off to the dragstrip to thrash on my car some more! (and
hopefully win some money too!)

Doesn't GM make new piston rings specifically designed to fix the oil burn issue?

Maybe he doesn't want to spend the money on it.
GM supposedly fixed the piston rings in 2002, yet I don't believe it, since I know a few 2002
owners (Vettes and F-bodies) that have consumption and piston slap problems. As a matter
of fact, one of my friends that I saw at the track today, is having major piston slap problems
with his 2002 Camaro SS. The car runs strong (he ran 13.3s in the heat and his car is bone
stock) but just makes a lot of racket. He says it sounds like a 1984 Chrysler Minivan!

PS-I made it to the finals in the bracket racing, but redlighted and handed the win to a good
friend of mine. I still won $100 for second place though! More money for my car stereo now!

Plus I made 10 quarter mile runs, putting this Maxlife oil to the stress test. We'll see how it
holds up on the next oil analysis.

[ August 10, 2002, 08:34 PM: Message edited by: Patman ]

Originally posted by MolaKule:
I wonder what would happen if he went to the Delvac 1 5W40? It stays pretty much SAE 40.

I'll suggest it to him, but I think he might be scared off by the fact that it's meant for diesels.
I'll try again to see if he might consider using a 15w40 instead, such as Schaeffers.
Yes, from everything I have heard, the additive package is different (and not emission-friendly) in the Mobil 1 motorcycle oils ... and 50% more expensive as well.

I'm skeptical that such a slight increase in weight could affect oil consumption significantly. Maybe it's me but 12W38 (computed by weighted average) doesn't seem like any kind of "magical bullet" for combating oil consumption. Did this guy measure precisely??

Although, I suspect the practice of mixing the way he did it is fairly harmless. Keeping within the same type & brand almost always is.

Patman, the guys with piston slap, have they tried a oil with moly in it?

--- Bror Jace
Now you understand why Mobil Oz don't import M1 0/5W30. Valvoline only import MaxLife 20W50. Same engines/weather here as in the US, that should tell you something?
Bror, I have suggested that those with piston slap try out an oil with moly, and many have
switched to Redline and found it cured the problem, or reduced it. Some people have it so
bad though that nothing will help. Those are the guys that typically end up with new engines.

Originally posted by sprintman:
Now you understand why Mobil Oz don't import M1 0/5W30. Valvoline only import MaxLife 20W50. Same engines/weather here as in the US, that should tell you something?

Yep, but I don't think the thicker 50wt oils are a safe choice. For most cars I still think that a
10w30 or a 15w40 is all they need.

How would moly reduce piston slap? Moly plating on the cylinder walls?

I thought that piston slap was due to clearance of the piston in the cylinder and the "slap" was the noise of the piston rocking back and forth in the bore.

Can anyone explain what piston slap really is?

My old BMW has some noise when stone cold that I thought was piston slap. Once the car warms up. the noise goes away and I visualized the pistons expanding to fit the bore. Am I crazy?

Don you are correct re piston expanding. Traditionally cast or Hypereutectic type run tighter piston to wall than forged but that has changed a little and these days forged run a tad tighter. One of my 351 Clevo's ran just under 7 thou piston-to-wall and did it rattle when cold.
The little bit of extra thickness of the moly may be responsible for reducing the slap, or simply
the lesser metal to metal contact. I'm really not sure. But our own Bror Jace on here found
he was able to elminate the piston slap on his Honda by switching to Redline, which has a lot
of moly.
A quote from Heywood's book on Internal combustion Engines regarding piston friction:

"The side thrust (force) [equation referred to]is transmitted to the liner [cylinder wall] via the rings and piston skirt. It changes direction as the piston passes through top- and bottom-center positions. Since the friction force changes sign at these locations and the gas pressure during expansion is greater than during compression, the side thrust during expansion is greater. The psiton skirt carries part of this side thrust [force] so it contributes to piston assembly friction. The large contact area between the skirt and liner, relative to the ring contact area, results in higher loading [pressures] and promotes hydrodynmaic lubrication. Piston skirt areas have been reduced substantially in recent years to reduce piston mass(which reduces side thrust) and contact area. An additional reduction in side thrust, leading to reduced skirt friction, has been achieved with the use of an offset wrist-pin. By offsetting the pin axis by 1-2 mm without changing the vertical location, the crank angle at which the piston traverses the bore and "slaps" the other side of the cylinder is advanced so it occurs before combustion has increased the cylinder gas pressure significantly."
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