Thin oil question

[addyguy]

Originally Posted By: Ken2
10W-40 oil is bad news. Always has been. If it is conventional oil, it has too much viscosity spread (30 points) and that requires too high a proportion of viscosity index improver polymers. These do not lubricate. They make the oil thick when the viscosity is tested hot flowing through an orifice, but the oil does not protect as well as an oil truly this viscosity without the VIIs. When the VIIs shear, you don't even have the illusion of the higher viscosity.

Avoid 10W-40. If you feel the need for a 40wt oil, get a 15W-40 or 5W-40. If all your engine needs is a 30wt oil, get any of the many top quality 10W-30 or 5W-30.


In the 1970's, that may have been true.

Today, it is not. Modern 10W-40's are excellent oils for certain applications.

Look on any PDS sheet - 10W-40's and 5W-30's often have close to the same viscosity index. BUT, I'll bet the 5W-30 uses MORE VII's, as it starts with a thinner base oil.

By your logic, that should make 5W-30 'bad news' LOL!

 

[BuickGN]

Originally Posted By: demarpaint
Originally Posted By: ekpolk

But there are bearings with tight clearances, tightly fitted pistons/rings, and so forth.



+1 and in some engines pumps designed specifically for the thinner oils.

Clearances are no tighter today than they were 50 years ago. That myth or assumption is one of the most prevalent around here right next to the nooks and crannies.

Again, no problem here with my 20w-50 all year round. Going by memory, my cam bearing clearance is the tightest clearance in the engine at .001" and the cam bearings have always looked great.

I would like to see a modern engine with a clearance of less than .001" anywhere.

Rings don't have clearance to the cylinders, never have. Pistons do but hot clearance is not that tight.

The best argument I can see would be not running the pump at it's relief psi as much with a thinner oil.

 

[Gary Allan]

The thrust of the movement toward thinner oils is lowering the power to pump it through the engine. You're simulating a starting condition that a heavier fluid experiences much later in the warm up process.

 

[aquariuscsm]

On the 10W40 note,wouldn`t a syn 10W40 be more stable than a syn 0W40 via less vis spread?