Why oils thicken over time ?

[seremban]

Hello ... I'm new here. Anyway I have read some comments in this forum stating that normal pattern for motor oil is the oil will shear and become thinner, then thickening over time.

Question : why does the oil thicken gradually ? Does this apply to multi grade oils only or both multi and monograde oils ?

Thank You.

 

[vxcalais]

Oxidation ? Build up of broken down particles, additives, soot and gunk trapped by the oil doing its job etc. Therefore will build up in the body of the oil, thickening it. Also further into the drain interval, viscosity modifiers may be totally over used and not work anymore, leaving the oil not capable of achieving its designed viscosity. It will thin initially, then thicken up.

 

[ADFD1]

In laymans terms, they become spent.

 

[Boomer]

In chemical terms, you get the formation of reactive intermediates such as free radicals, that combine and produce molecular species of higher molecular length and weight. We perceive this on our scale as a more viscous, thicker liquid. Antioxidants in the oil as well as oils with fewer reactive sites (synthetics) help to prevent and /or slow this process down. With modern refining techniques, there are fewer rective sites in even dino oils so this does not happen as fast as it would have decades ago.

 

[Tom NJ]

Originally Posted By: Boomer
In chemical terms, you get the formation of reactive intermediates such as free radicals, that combine and produce molecular species of higher molecular length and weight. We perceive this on our scale as a more viscous, thicker liquid. Antioxidants in the oil as well as oils with fewer reactive sites (synthetics) help to prevent and /or slow this process down. With modern refining techniques, there are fewer rective sites in even dino oils so this does not happen as fast as it would have decades ago.

Perfect explanation - spot on!

 

[CharBaby]

Originally Posted By: ADFD1
In laymans terms, they become spent.


Indeed!
But here's another layman way of looking at oil and how it thickens. As Thanksgiving has just past, remember how your mother, wife or even yourself would make the gravy for Thanksgiving dinner. Take the juice from the turkey(the juice being the oil) and keep adding flour while mixing. See how it thickens? Of course this is an exaggeration to the extreme but, I think it explains the process. Hmmmmm, kinda makes me hungry again!

 

[eoghan]

in addition to the build up problem already mentioned there is one more thing as well and that is over time engine heat will burn off the more volatile lighter compounds in the oil leaving a much thicker oil in the process.

This is the Noack Volatility ratings that you see on oil labels!!

 

[ZZman]

As mentioned above:

Build up of contaminants
Lighter molecules in oil evaporate/burn off leaving heavier ones behind.(Noack)

Good Syn oil help resist this.

 

[21Rouge]

Originally Posted By: ZZman
Good Syn oil help resist this.


It seems easier for good synthetics to resist thickening but not so much thinning? The current example is M1 0W40.

 

[cjhepburn]

What would make m1 0w-40 thin? Is it fuel contamination and if so that would impact any oil.

 

[21Rouge]

Originally Posted By: cjhepburn
What would make m1 0w-40 thin? Is it fuel contamination and if so that would impact any oil.


For sure fuel seems the cause of much thinning. I just wonder why there isnt a formulation which helps PCMO oils withstand the stress arising from the presence of fuel. Looking at so many UOAs it seems to be a pervasive problem.

 

[ottotheclown]

Duh, just do short oci and this problem will be reduced to the point your motor will survive.

 

[21Rouge]

Originally Posted By: ottotheclown
Duh, just do short oci and this problem will be reduced to the point your motor will survive.


Of course. But my question was why there isnt (or maybe there is) a formulation or additives which helps keep an oil in grade in the presence of gasoline.

 

[JAG]

Originally Posted By: 21Rouge
Originally Posted By: ZZman
Good Syn oil help resist this.


It seems easier for good synthetics to resist thickening but not so much thinning? The current example is M1 0W40.

The example is a very wide spread oil which is why it thins quite a bit. To compare synthetic vs. dino with respect to shear stability at least compare same viscosity grade.

No additives can prevent thinning from fuel dilution. It's a simple matter of mixing a thin hydrocarbon fluid with a thicker hydrocarbon fluid where it is inevitable that the mix will be thinner.

 

[db500]

the goobledeegock gets gooey over time...that's the scientific reason.