Thickening versus thinning?

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Sep 25, 2002
Loveland, Colorado
Question: Is it better if an oil thickens to the same viscosity that another thins? [I dont know] For example, if you have a an oil that starts at 11cSt but thickens to 13cSt, & another oil the starts at 15cSt but thins to 13cSt, do you want the one that thickens? The reason I'm asking is because I thought any thickening was the result of oxidation, & that there was no additional benefit in protection from this increase in viscosity.
Very timely question for me. I just had a UOA done on Shell Rotella 5W40 after 7,200 miles. The lab said it had thickened to what amounted to a 50 weight. They said there are traces of fuel in the oil, and they attributed the increased viscostity to the fuel. I would think the volatile part of the gasoline flashes off, leaving solids in the oil, which, I assume, results in the increased viscosity. Certainly this is not going to provide increased protection (quite the contrary)! At the 7,200 mile mark, the wear numbers were very good. So I also am interested in what anyone has to say about oil thickening.
Why would the lab think that fuel in the oil would cause it to thicken up? Fuel is going to thin the oil out.
I agree with Eiron...any signficant change in viscosity is a sign that the protection in the oil is no longer there. Thinning can be from fuel dilution or from shearing of the viscosity index improvers. Thickening can be from oxidation or soot loading in a diesel. All indicate the end of the oil's life. Fuel dilution causing oil thickening??? I've only seen the opposite. Ken
Originally posted by Patman: Why would the lab think that fuel in the oil would cause it to thicken up? Fuel is going to thin the oil out.
Intuitively that's what I thought too. Here's a quote from the Blackstone report: "One minor problem found was in the mild gas dilution and a resulting higher viscosity." They listed fuel % as "TR" for trace. Insolubles were 0.6% agaist the lab's standard of <0.7% [at 7,200 miles on the oil]. Viscosity of this 5W40 oil tested at 85.5! The "Ask the expert" at Shell's website came back with this: "Detergent action from ROTELLA T Synthetic may have cleaned up deposits and hence the oil accumulated solids, thus thickening the oil. The presence of fuel may also indicate higher than normal blowby, which may also contribute to solid loading of the oil." And our own DrStressor [member 888] came up with this: "Both the increased solids showing up in the Blackstone analysis and the viscosity increase can be the result of fuel dilution. While dilution of oil with gasoline should DECREASE the viscosity, longer term exposure to fuel will accelerate oxidation. In the case of synthetic oils [Group III in this case], where the base stock is very oxidation resistant, the particulates usually come from the residue of the fuel itself. So the viscosity of an oil from an engine experiencing fuel dilution typically depends on how the car was driven right before the sample was taken. If the engine was run hard enough to drive off most of the fuel, the viscosity of the oil can show an increase as the result of accumulation of oxidation products [solids]..." So that's why I now believe that fuel dilution CAN result in higher viscosity. [ September 02, 2003, 07:37 PM: Message edited by: CJH ]
Guys, Thanks for the replies! I find it interesting that fuel contamination can actually increase viscosity, but Shell's & DrStressor's answers make sense. I'm guessing oxidation & solids-loading is in no way beneficial to lubricity. So is this the answer to my question? Is an oil that thickens from 11cSt to 13cSt a worse choice than an oil that thins from 15cSt to 13cSt? Does this mean the thinned oil is providing more protection (at the same viscosity) than the thickened oil?
It appears that something other than normal wear and tear, such as fuel contamination, in the oil causes it to thicken. This is added contaminants in the oil. An oil that thins is usually due to "normal" wear and tear on the oil. It's doing it's job. I would think that it is better to have an oil thin a bit with less contaminants than thicken with additional contaminants. Of course ideally the oil should not thicken or thin out during it's use. [Big Grin] Whimsey
It may be possible to design a very cheap engine oil to resist viscosity changes. Match the thinning trend of the lubricant with volitile base lubes is one method. That way the thinning affect from degradation is balanced by the thickening effect caused by boil-off and oxidation. One top tech guy from a large oil company told me that this is one example of the smoke and mirror nature of the lubricant business. A good blender can make an engine oil do whatever he/she wants. From reading the above thread, I came to the conclusion that the state of tune of an engine, how the vehicle was driven, engine design, the length of trip just before a sample was taken, and a host of other un-controlled variables, can also make a lubricant look good or bad in a UOA. [ September 04, 2003, 03:41 PM: Message edited by: userfriendly ]
I think I agree with Whimsey. It sounds like the oil that thins from 15cSt to 13cSt has fewer "nasties" than the oil that thickens from 11cSt to 13cSt. (Am I correct that I'm agreeing with you?) userfriendly, did the "tech guy" you spoke with mention anything about how the causes of viscosity changes affect an oil's performance? Is there anyone with data (MolaKule? Others?) that supports one direction in viscosity shift over another?
Eiron; No, I think the topic of grade reduction came out of me mentioning that in the 60s and 70s I noticed that after about 1500 miles of use, the oil consumption suddenly went up in vehicles that I had owned. Later on in the 80s I owned several vehicles that were equipped with mechanical oil pressure guages. I noticed that the oil pressure also dropped after a short service interval, that coincided with the sudden oil consumption issue. I mentioned that after reading several UOAs, here and on other boards, some engine oils thickened, some stayed the same, while others thinned. That is when the topic of blending tricks came up. .......And there is nothing lke repeating gossip.
Some info just in from Blackstone (excerts from an email): Fuel gets into the oil, thinning it and causing the oil to run warmer, which in turn, cooks the fuel back out taking light fractions of the oil with it. Raw, volatile fuel in the oil will cause a lower viscosity. If it has been in there for awhile, it isn't unusual to find a high viscosity along with higher than normalinsolubles. We can see the diesel-use oil. It is possible, as you suggest, that the oil is picking up some residual sludge. If this is the case, insolubles should drop right back down in the next sample.
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