This question is for DrDevon, or anyone else that is very familiar with oil and it's properties. I was told a number of years ago from a well known engine builder, that after engine oil became "dirty" (I don't know how to quantify this term, but would assume that this means after running a couple of thousand miles) that multi-viscosity oils lose their ability to change viscosity, and remain fixed at their 2nd number.
I.E. if you were using say a 5W-30 oil in your truck, this engine builder was saying that after "x" number of miles, this oil would remain fixed at 30w and would stay at that viscosity.
Is this nonsense, or is there truth to this??? The engine builder, and rebuilder was saying that the combustion by-products taint the oil and keep it from changing viscosity, so if you were running a 10W-30, he told me to just run straight 30 weight. He claimed he did this all the time. I never followed his advice, but I've always wondered about his claim. Any info????

 

DrDevon has good information on oil. I am sure he will answer this one soon but until he does, because I don't know about this claim. Personally I think it is BS, but I will say this dont ever run just a 30wt oil or you will tear your engine up quick. It wouldnt be so bad if you never had to start it. It would take some time for a plain 30wt to lube your motor at start up and until it started to lube it there would be ALOT of wear.

 

I would disagree with the mechanic on this one. Oils now days are made to be changed at 5,000 miles for normal everyday driving, even though most people have gone to 3,000 mile drain intervals (this was brought about by the Quicklube market), so it would be a very rare instance that an oil would be severly broken down enough in a limited time to cause it not to function properly. How does this mechanic determine when an oil is "dirty"? Unless he is doing oil analysis their is no way he can determine this.

Additive packages are blended into a base stock to make an engine oil and give us the protection that is needed. The base oil is what lubricates the engine, and the additive package is basically what allows the base oil to lubricate. Their are detergant dispersants, anti-wear additives, polymers (which allows an oil to be a multi vis) and other additives in the motor oil to help protect the base oil and allow it to lubricate. One indication of a quality of an oil is the viscosity index. The higher the number usually indicates a better oil to a certain extent. As long as you you use a quality oil and have a proper maintenance program, you will be fine.

 

Good to see you again Navi Man. So where is this damn Dr. when we want a good argument, I mean good debate... he will however have some good information for Don25.

I like your post better then mine. Makes mine kind of suck