best overall viscosity

[MolaKule]

Regarding SAE30 and SAE40 for 2-cyle diesels:
Straight weights were or are used because these engines needed low-ash lubricants, not obtainable with multi-vis oils. The multivis additives at one time had high-ash (metallic) components which the 2-cycle diesels could not tolerate, specifically, the electromotive engines on trains.

Regarding Multivis oils in general:
Nornmally (at least for dino oils), the blender will start with the lighter viscosity base and add VII's until the spread is acceptable. For example with a 10W40, a 10W viscosity is used as the starting fluid and VII's are added in proportion to get up to the 40W classification for high temperature operation.

Multivis (true) synthetics have such stable and wide VI bases that only a small amount of VII is needed to obtain the needed viscosity spread.

 

[widman]

We are starting to confuse things a little. The Detroit Diesel's recommendation for straight weight oils have to do with engines that shear oil, so to use a multigrade it really has to be one that holds up better than most. I have 15w40 oils with 0.50% sulfated ash and others with 0.90%.
On formulation, Bror Jace is right that you start out with the lower viscosity base oil and add VI to get where you want it. With synthetics you just add less, or sometimes probably none. There are cedrtain esters that are good to combine with PAO's for anti-wear characteristics, but they have 0 VI. That's another subject. As is the decision as to which VI improver to use. If you use one with a real high resistance to shear, your oil will thicken out of grade with oxidation. If you use one that shears too fast, or your oil never oxidizes, it will thin out too fast and fall out of grade.
To just call a synthetic multigrade a straight weight is legal but incomplete. We complain enough about the companies that don't tell you how their "Synthetic" is formulated, let's not suggest that they just call it by the 100 C weight and leave us guessing as to it's cold temp performance. If it's viscosity at low temp meets the 0w, 5w, 10w, or whatever, why not say it and let people know?
The Airplane recommendations for straight weight also say this is for people who use them less than 100 hours a year. That means they probably sit for weeks without use, with the oil draining off. It also mentions preheating the engine. How many of us are willing to do that every morning.
I have a study that went on at Cummins between 1989 and 1998 that shows that when equally formulated CF-4 oils were used in diesel trucks, when idling a 40 weight oil used 40% more oil than a 15w40, at cruising 180% more, and under high load 32% more than a 15w40. Combine that with those of us who start up at O F one day and 110 F the next, just to go back to 0, and we have a good use for multigrades.

 

[GW]

Molakula - thanks for pointing out that Detroits require a low-ash oil, I had forgotten about that. They also do specify in their Fuels and Lubricants manual that if a multi-viscosity oil is to be used, (not recommended) it must meet a required minimum level in a shear-stability test. (I believe it is >3.7cP)

Quote from DDC:

"Single Grade-High Ash Content Lube Oils
High ash oils (greater than 1,000 mass % ASTM D-874) tend to form excessive deposits on engine parts, resulting in impairment of their function (exhaust valve closure, for example). These are NOT recommended, except where the continuous use of high sulfur content diesel fuels (greater than 0.5 mass %) is unavoidable and where a high TBN, LOW ASH oil is not available.

Multi-Vis Oils
Less than optimum engine life must be expected when using 15w-40 oils. Multi-vis oils tend to break down due to mechanical and thermal stress which results in a loss of viscosity and premature engine wear. Multi-vis oils do help cranking when temperatures are below 32 degrees F (0 degrees C). However, once the engine starts, the major advantage of the multi-vis oils is exhausted. When the engine reaches operating temperature, the internal engine temperatures and pressures vary little from an engine operating in much warmer climates. This is when the 2 cycle engine benefits from the superior protection of a 40 wt."

Widman - your are correct that they mentioned preheating the oil in that article but.....would that be necessary when using a synthetic SAE 30 that has a lower vicsosity @ 40C than a 10W40 and with a pour point that is the same? What are your thoughts on the SAE study that showed lowest wear with an SAE 30 as compared with a 10W30?

Can anyone comment on the possible consequences of using using a high-ash diesel (1.5%) formulation in an older gasoline engine? Would they not also be prone to deposit formation?

[ June 10, 2002, 11:24 AM: Message edited by: GW ]

 

[widman]

GW- I don't recall a SAE study on 30 vs 10w30... There was the article from the airplane mechanics, and I agree that a thicker oil will stay on parts longer and therefore protect when not used often.
The problem with straight weight oils is that there is no specific low temp spec. they have to be between 9.3 and 12.49999 at 100 C. But there are no lower limits. A synthetic will probably be better than others as far as the range is concerned, but one would have to check the specific data sheets for a specific product. Even then some of the sheets are so out of date that they might be wrong. If they have better low temp capabilities, they should at least say it. Even if to say it is a 25w30. That would tell you at -5 you would have 6000 cP cranking visc. and at -15 you would have 60,000 cP pumping.
As for the high ash.... I haven't seen studies on it in gasoline engines, but from all I have I would never use more than 1.40 in a Diesel. Normally gasoline engines have no need for all the high ash detergents.

 

[GW]

Here is what was said about the SAE Paper in that article:

Last March, a pair of Mercedes-Benz Engineers (Rudolf Thom and Karl Kollman)along with pair of Shell Oil engineers (Wolfgang Warnecke and Mike Frend)wrote an SAE technical paper (No. 951035) on "Extended Oil Drain Intervals: Conservation of Resources, or Reduction of Engine Life?" Among the many interesting results presented in the paper was a graph showing cylinder-wall wear-rate Measurements correlated with cylinder wall temperature. Cylinder wear rates(in micrograms per hour) were monitored as a function of cylinder-wall temperature in a test engine specifically fitted with radionuclide-impregnated cylinder liners. Three test were done so that three different kinds of oils could be used. In one test sequence, a straight 30 weight oil was used; in another, 10W-30 multigrade; and in the third, straight 10 weight oil. (The test engine was a Mercedes-Benz OM 616 2.4-liter, 4cylinder diesel.) In each case, the engine was operated at fixed speed, torque and temperature conditions until constant wear rates were observed. Wear rates were then plotted against cylinder wall temperature.

The graph of cylinder-wall wear rate versus cylinder wall temperature tends to be bathtub-shaped, with wear increasing sharply at each temperature extreme (as you'd expect). But while two of the oils turned in very similar wear performance, one oil stood out as protecting the engine against wear at the extremes of temperature. That oil was plain SAE 30 (Straight-grade 30-weight). At either extreme of temperature, the maximum wear rate with 10W-30 was more than double that of the straight SAE30 oil. (The worst performance was turned in by straight 10-weight.)

These findings should come as no surprise, sincein generalthicker oils make for thicker oil films, and the thicker the oil film, the better the wear protection. What's surprising is that a 10W-30 oil (which is supposed to have viscosity comparable to an SAE 30 oil at high temperatures)does not provide at least equal wear protection to that of a 30 weight unmodified oil. The simplest explanation, it appears, is that the base stock from which a 10W-30 multigrade is madenamely, 10 weight oil is not as good a lubricant as 30-weight base stock. Also it would appear that viscosity-index (VI) improvers are not, in themselves, robust lubricants.

 

[BOBISTHEOILGUY]

There is a very simple explaination as to why a st 30wt will protect better than a 10w30. It has nothing to directly do with how good the vi improvers are.

VI's are designed as such that when added to a base oil of say a 10wt, it will expand when heated, thus taking on the flow charateristics of a 30wt... so what happens when put in a shear zone ? Answer... The VI's are squeezed and will collapse back to the lower viscosity or the base wt.. in this case a 10wt oil.

So if the oil is a 10wt oil in the high shear zone, and the st 30wt has no vi's and will not collapse out of grade, which has a higher film hydrodynamic film? Of course the st 30wt.

This is one reason a good barrier additive is needed so to compensate for the lower hydrodynamic film.

[ June 10, 2002, 10:09 PM: Message edited by: BOBISTHEOILGUY ]

 

[Bror Jace]

Also Bob, don't forget that viscosity improvers by themselves supposedly have no lubricating ability.

I forgot where I read that ... Red Line's site?

 

[JTC]

Re VI improvers & high pressure shearing effects on wear, I wonder if the supersyn used in the new Mobil 1, which seems to act both as a lubricant and a VI improver, would be giving that extra bit of protection touted by the Mobil marketing materials. Maybe they have managed to produce a lubricant which does not shear back to its base weight; maybe the supersyn both lubricates and limits shearing, keeping the lubricant closer to its 100° viscosity.

Just speculation.

JTC