what is it exactly?

 

Thinner!

Ford and Honda use it. Ford uses it since they're backwards and Honda uses it in the S2000 so it can rev to 40 billion rpm and make 105 hp.

I'm not biased or anything

 

i know that

 

It's just a thinner oil.. Different engines are built with different clearances, and as such they call for different oils. The tighter an engine is built, the thinner the oil has to be to properly get in between every nook & cranny. On the other hand, I've seen some German engines that are built so loose they require 20w50 from the factory. Also, obviously, as an engine wears down, some of the clearances will increase, so a thicker oil could be used.

 

The basics:

The number preceding the "W" is the cold temperature viscosity, measured in centistokes at zero degrees F. An oil must fall between a certain centistoke range to be rated as SAE 0W, 5W, 10W, etc.

The last number, or only number if no "W", is the high temperature viscosity, measured in centistokes at 200 degrees F. An oil must fall between a certain centistoke range to be rated SAE 20, 30, 40, etc.

In order to be rated as multi-viscosity, an oil must meet both ranges at the specified temperatures.

So, a 5W-30 and 5W-20 would be the same viscosity at zero degrees F, but the 5W-20 would tend to be thinner at 200 degrees F (there is some overlap between the rating ranges, so they could actually be the very same oil).

Manufacturers are recommending the 5W-20 for increased fuel economy. End of reason.

Now, a few basics on multi-viscosity oil formulation: A polymer, called "viscosity index improver", or VI improver, is added to the oil. This polymer expands with heat, taking up more space and therefore keeping the oil thicker than it would be without the additive. These additives are typically the first thing to break down with heat and stress in an oil, so the less of them you have in the oil, the better the oil will lubricate at higher temps and the less they will form sludge and varnish at higher temps.

FWIW, synthetic bases are typically more naturally multi-viscosity, so very little or no VI improvers need to be added to accomplish a multi-viscosity rating.

 

Ok--say you need to add a quart or 2 from being low--but all you have is diesel oil--could that hurt your engine? Or would it be ok--in a pinch--not making a habit out of it or anything.

 

What 5-7 is saying is that 5W-20 (or anything "W"-anything mineral oil) is basically 5W oil (ISO 22) which has been fortified with expendible, long chain polymer molecules. The same is true for any multigrade. The "W" rating is the base oil stock viscosity rating. The high temperature viscosity rating is achieved by the addition of fortifiers. The greater the difference between the "W" number and the hot number, the greater the amount of fortifiers. This is not usually a good thing, so a 5W-20 is probably better than a 5W-30 for a significant reason.

These fortifying molecules tend actually curl up and become slightly smaller at lower temperatures, occupying less space and acting like less viscous oil molecules. They also tend to elongate and straighten out at increased temperatures, taking up more space and acting like more viscous oil molecules. As 5-7 so eloquently explained, these molecues are the first to wear out and break down under stress, leaving you with little more than 5W oil in the sump. Worse yet, it is precisely these long chain polymer additives that burn readily and create ash and varnish in your engine, so the more fortifiers that are added typically means more sludge, varnish, and coking/carbon in the engine. It is for a large part the oil that contaminates itself, not combustion byproducts as you might presume. Add some atmospheric moisture to this soup in the sump and you'll start forming destructive acids that attack iron molecules and soft metals (like bearing shells), making them etch and wear even faster. In many cases, multigrade oils don't pass the hot number viscosity cup tests after only a thousand miles, yet most people drive at least 3,000 or more miles between changes, most of them on basically 5W oil, and have no clue about what is really left in the sump to "protect" the engine.

Synthetics "cheat" the SAE grading system by remaining at a near constant viscosity over a wide range of temperatures. Since almost all the molecules are synthesized, they don't break down nearly as readily as the additives in mineral based multigrade oils. Because of this, thermal viscosity breakdown is essentially eliminated, and the engine gets relatively "thin" oil at low temperatures and adequate viscosity at high tremperatures - something conventional multigrades try deperately to do, but always fail after time. If you like replacing bearings and rings, don't bother with synthetics - just collect the money you saved on oil for new parts. If you are tired of repairing engines, consider synthetics.

(BTW - You had to know this was coming...)

 

Generally, it's not good to mix brands, weights, or types of oils, simply because each is formulated to provide what it does with its own package - not necessarily with what's in the other guy's/type's package.

But, in a pinch, they should be compatible. You should, however, use diesel oil that also has a "spark ignition" rating. Look for "API SJ, CE" or something to that effect. The "Sx" rating is gasoline engines (spark ignition), and the "Cx" rating is diesel (compression igition).

 

my car burns lots of oil, I justmped from 10w-30 to 20w-50 to try and slow it down....I need this engine for about 6,000 miles......even tho i am adding lots of oil stuff still builds up right?