Ural Oils

Urals have a roller bearing crank and gear driven cam. This applies massive shear loads to any oil in the engine. Oil lubricates by forming a film on the surface to stop metal to metal contact. It also acts as a coolant by taking heat from the hottest parts of the engine such as the underside of the piston crown and the area around the exhaust valve and carrying this heat to the sump where it is cooled by air flow. To achieve lubrication it is vital that any long chain molecules in the oil can not be sheared (or cut up) into short chains.
As stated the cam in the Ural is gear driven and extreme pressures exist between the teeth. Roller bearings found in the crank big end of Urals which are constantly accelerating and then slowing apply huge shear loads to oil. From this you will see the oil required for the Ural must be able to withstand shear and pressure loads much greater than those found in a plain bearing crank with a chain driven cam. Monograde oils are almost impossible to shear. A straight 40 will remain a straight 40 for the life of the oil. Mineral multigrades shear very easily and at very low pressures. A mineral 20w50 multigrade will shear to a monograde 20. Once this happens metal to metal contact will occur leading to fast wear rates and a very rapid increase in spot temperatures. Semi synthetic oils have long chain viscosity improvers which withstand shear loads and keep the viscosity grading correct for a longer period of time. Fully synthetic oils are even stronger than semi synthetics and are extremely difficult to shear, they will also withstand greater pressures. EP mineral oils for gearboxes have an extreme pressure additive to cope with the high pressures found between the teeth of the gears (EP stands for extreme pressure). There are several types of viscosity improvers, the most shear stable (as used in quality brands) are referred to as type VI. The cheaper semi synthetic and even a few fully synthetics use a low cost type VII improver that does shear.