Ok, where to start.
Let's start with the 10W/40 VS a 5W/50 oil. Ideally the closer you can keep the cold/hot viscosity range the better. The wider the spread requires more visosity improver additives, oils with a large viscosity spread tend to "shear" down more quickly then oils with a close viscosity range. In a perfect world your oil would only ever operate at one temperature and the only thing you would need would be a single viscosity oil like 30W or 40W. But since equipment, motors, etc. tend to start out cold and over time warm up to operating temp. a multi-viscosity oil is prefered. Multi-viscosity oils act like a lower viscosity oil at low temps and act like a thicker oil at operating temp. Synthetic oils have a greater temperature operating range and therefore need less VI additives.
So what's this mean.
Hydraulic oils are usually single viscosity like ISO 32 (SAE 10), ISO 68 (SAE 20), etc. Hydraulic systems normally operate in a rather narrow temperature range so a single viscosity fluid works fine and not needing VI additives. They tend to be the cheapest.
UTF's (Universal Tractor Fluid) on the other hand are for the most part a multi-viscosity oil. Knowing that equipment may operate anywhere from very cold to very hot requires a multi-viscosity oil to provide acceptable performance out of equipment. Otherwise in very cold temps hydraulic systems would operate very slowly and at greater preasures and in hot temps it would be to thin possibly causing a loss of hydraulic preasure.
"Do we really need the additives that real hydraulic fliud offers when synthetic motor oil is so slippery, anyway? "
That's not really the case. For the most part synthetic's are not that much more "slippery". It's a common misconception. Of all the synthetic base stocks ester would be the slickest but it is rarely used in large enough quantities to make a difference. The majority of synthetic oils are either GPIII dino based synthetic's or GPIV PAO's. Also the synthetic engine oil is still a engine oil that does not have the additives that a hydraulic oil or UTF has.
I'll touch on the additive difference in motor oils, hydraulic oils and UTF's.
Motor oils have large detergent additive packages to deal with sludge formation and to help keep these particles in suspenssion so they can be filtered out by the oil filter or drained when the oil is changed. They also have additive packages that deal with combustion byproducts (unburnt fuel), combustion gases and the acid's caused by these gases. There is a very large spectrum of additives used for anti-wear chacteristics. The only problem is that there is only so much anti-wear additives you can add to a engine oil without effecting the emission's system. ZDDP (zinc) levels have been going down in engine oils over the years due to the possibility of plugging up cat's and particluate filters.
Hydraulic oils have additive packages that are alot different than engine oils. There are anti-foam additives, greatly increased anti-wear additive levels, usually have anti-rust additives, other additives help deal with chemical breakdown caused by contaminates like water and air which cause oxidation and usually some type of seal conditioner to help protect seals.
UTF (Universal Tractor Fluids) are basically a hydraulic oil with a greatly boosted additive package. Additional additives found in UTF are greatly increased levels of extreme preasure additives to help protect gears, additives to help prevent chatter in wet clutch and wet brake systems and prevent slippage.
So which is best for a hydraulic system, specifically a Power Trac? Well, engine oils are engineered for use in engines. If they were formulated for use as hydraulic oil they would be called engine/hydraulic oil. Hydraulic oils are just that, hydraulic oils. They operate best in a narrow temperature range due to being single viscosity (except some specialty hyd oils used in industry). UTF would be my choice. It has a excellent additive package although some of the additives will serve no purpose in a hydraulic system. My understanding is that the Power Trac hydraulic systems tend to run a little on the hot side. One of the major benefit's of a UTF is that it can deal with a larger temperature range since most are multi-viscosity. The UTF fluid would offer the greatest protection and best performance characteristics of the three different types
It has been mentioned that some heavy equipment calls for motor oil or to use motor oil when hydraulic oil is not available. That's true but look at what API rating they are calling for, usually somewhere around a SE API rated oil. The older engine oils had greatly increased levels of zinc and other anti-wear additives that have been severly reduced in modern engine oils.
