Ok...lets make this easy...oil is used as much for cooling as for lube.
When they took sulfur and lead out of the gas it meant a bit less lube for the engine (both are very good lubricants). They used CASTOR oil in cars and plane engines...and that's a lot thinner than 5-20 oil.
HERE IS THE BEST I HAVE FOUND>>>
The Liquid That Acts Like a Solid
Oil has several purposes in a machine. It lubricates, cleans, and cools, carrying heat and debris away from surfaces of the parts it lubricates. Though a machine such as an internal combustion engine is made of metal parts, none of the parts actually touch, thanks to the oil. (If you hear a ticking or knock from your engine, that means parts are probably overcoming the lubricant and touching, and the engine may be headed toward failure.)
Oil, like any liquid, is non-compressible. It is this property that allows hydraulic systems to work. If you put oil in a closed vessel like a cylinder and press down on it with a piston, the oil will apply equal pressure to all the surfaces containing it. You can apply as much pressure as you want, but the oil will not compress. In this respect, the oil acts like a solid: although it is constantly flowing, it serves as a "wall" between the machine's parts, keeping them from touching.
Imagine the oil in your engine. It's under pressure, so it's acting like a soft, solid cushion and keeping the crankshaft from touching the friction bearings in which it rotates. In this scenario, the "solid" nature of the oil can be helpful or harmful, depending on its condition. When we find wear metals in your oil, it's the oil that's causing the parts to wear, not (usually) contact between metal parts. If the cushion of oil between the rotating shaft and the bearings is clean, it's like a slick tongue of Teflon. If it's dirty, it acts more like an abrasive pad. That is why it is vital to keep your oil clean if you expect to get long life from an engine or any other type of oil-wetted machine.
Well, you might think, that is why we use oil filters. And indeed, oil filters do help keep the oil clean, but only a little. Early engines didn't have filtered systems, and there was debate for many years after they were introduced whether they were really necessary and whether people would be willing to pay for them. Filters can only filter out the largest particles from the oil. The finer particles pass on through and cause abrasion at the oil-wetted parts. Dirty oil in an unfiltered system could be thought of as a 50-grit pad at the bearings. Dirty oil in a filtered system would still be a 220-grit pad.
The Importance of Viscosity?
The viscosity, or thickness of the oil, is not nearly as important as many people think. Oil retains its nature no matter what thickness it is. Think about this: automakers are continually recommending lighter multi-grade oil in new engines. The reason is increased
efficiency.
It takes power to pump oil through an engine, and the lighter the oil, the less power required to pump it. The oil's ability to act like a solid and protect parts is not related to its thickness.
If that doesn't sound quite right, consider this: The gears in a heavy duty Allison automatic transmission are doing the same work as the same machine equipped with an Eaton manual transmission. Due to the hydraulics of the automatic, it runs on a 10W automatic transmission oil. But the manual transmission uses a very thick (sometimes up to 90W) gear lube oil. The gears of both types of transmissions will have a similar life span. We don't find any significant differences in wear, regardless of oil thickness.
Oil's Fourth Function
Oil is a wonderful substance that allows machines to work. It lubricates, cleans, and cools. If you use oil analysis, your oil has a fourth function too: The ability to provide information about how a system is wearing. Machines of similar manufacture will wear similarly. The wear levels present in a machine's oil can provide useful information about the unit's mechanical health. A poorly wearing machine will fail sooner. A really badly wearing machine may be in the process of failing, but if you catch the problem in time, you might be able to fix it, giving the engine new life.
Oil analysis is a relatively new technology compared to the many decades the industrial revolution has been with us. As its benefits become more clearly understood, we are only left to wonder, why didn't someone think of this sooner?
Question about Oil 
but I put it in their any way for the longevity . Being the chief oil change engineer in the family, I also switched my dad's stuff to synthetic oil. His 55 chev gets 10w-30 and his softtail deuce gets 20w-50. I know their not mustangs I just wanted to list an example of inhertience investment 
