Fuel reacts with Oil?

[MolaKule]

If the oil temp can get high enough to evaporate the dule, then I wouoldn't concern myself too much.

However, unburnt fuel and high fuel dilution of the oil can react with the oil and can lead to accelerated oxidation and thickening of the oil. Unburnt hydrocarbons add to the soot loading of the oil.

See this article:

http://theoildrop.server101.com/cgi/ultimatebb.cgi?ubb=get_topic;f=56;t=000017

 

[Ray H]

The matter isn't "either"/"or", it's "both". Some of the fuel is subject partial oxidation during hot conditions before it's harmlessly evaporated and recycled through the combustion chambers, burned and expelled through the exhaust system. This partial oxidative state happens in the combustion chambers, too, as evidenced by coke buildup on valve faces, piston crowns, and ring lands. Those oxidation products in oil, given the chance, will clump to form insolubles and/or directly further react with the oil forming more oxidation products leading to the varnish and sludge buildup seen in poorly maintained engines. Detergent/dispersants go a long way towards neutralizing and holding those nasties in harmless suspension. The formation of these oxidation products is unavoidable with any type of driving, but urban crawl, hot weather, and/or high output operation (all defined as "severe service") accelerate the process. Fuel oxidative products react especially well with sulfated and polycyclic aromatic contaminants rich in double bonds - neither of which are present in Group IV base oils, and nearly so in Group III base oils. Group I base oils have the highest amounts of these contaminants. But, given sufficient time in use, even synthetic oils will eventually start down the reactive path, too.

 

[Winston]

So, an oil with more detergent/dispersants is able to handle more of the oxidation products.(synthetic oils generally have more than dino oils right?)

What about the fact that dino oils have more misc. small chain molecules than synthetic oils. Does that affect its reactivity with fuel?

BTW, thanks for the two quick, authoritative replies. As well as the link.

 

[MolaKule]

Quality dispersants and esters go a long way to combat sludge buildup in both mineral and synthetic lubes.

Mineral oils have a mix of large and small molecules such that the smaller molecules are more volatile and burn off quicker, leaving behind the heavier, thicker molecules.

In a full synthetics with esters or Alkylated Naphthalenes, its the solvency that helps combat the agglomeration of sludge particles.

BTW, the cost of Alkylated Naphthalenes (AN's) do not provide any cost or tribological advantage over esters IMHO.

 

[Winston]

I understand that with a lot of short trips fuel will get in the oil(fuel dilution). I also understand that the fuel will evaporate out of the oil on trips where the oil warms up sufficiently.

Anyway, my question is; does the fuel "react" with the oil making other contaminants? Causing varnish or other bad compounds that do not evaporate out of the oil?

Also, does fuel react differently with dino vs. synthetic oil.