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.
I contacted various oil companies and asked them to recommend oil based on our engine but without telling them the make. I think if you tell them the make they just look it up in an out of date list probably compiled before fully synthetics were widely available. I asked them to recommend oil that would be shear stable in a roller bearing engine but could be used over a wide temperature range. I also said if there really is no oil with shear stability to match mono grades they should tell me so I could correct my current recommendations. I told them the engine size, type, bearing type, pump type, max power, max revs. A summery of their replies follows:
With roller bearing cranks, you need an oil made with large molecule bases. One needs to be careful not to go too thick as it then prevents rapid circulation at cold start - which is when 70% of engine wear takes place.
Viscosity index improves; the polymers that make a base material into a multigrade vary enormously in their shear stability. We at Rock use an Isoprene Diene type, the most shear stable. Synthetics offer a demonstrable benefit in all areas of lubrication compared with mineral oils. They have a higher natural VI and are more shear stable.
I would definitely recommend Rock Oil Synthesis 4 Racing. A 15w50 fully synthetic oil with API SL, ACEA A3, and JASO Ma specifications. We also have an interesting 20w6O racing oil called TRM which I would recommend for any older engines. We use TRM for endurance racing in roller bearing cranked bikes, with phenomenal results.
Drain intervals are always tricky. As an oil Company, we are always advised to follow the manufacturers recommendation. There are many factors that influence drain intervals. Older type engines tend to produce more or dirtier by-products from the combustion process. However, for a good synthetic oil, your 2500 km seems too short. I would feel confident at double that.
Based on the info provided we would suggest our Formula RS 10w60. This is a fully synthetic oil using highly shear stable VI improvers. It is only the VII's that are prone to shearing permanently. (Castrol) Formula RS is our flagship brand and uses VI.
20W/50 grades tend to be formulated with mineral oils that provide a good oil film, ideal for roller bearings. They exhibit minimal shear thinning. 20W grades will also provide good cold starting down to -15 degrees C, so only really cold weather may be a problem. In conclusion I would recommend the following product from our range: Morris V-Twin 20W/50. Do not be put off by the name; this oil is suitable for motorcycle engines that are not a V-Twin configuration. 20W/50 engine oils, like V-Twin 20W/50, tend to be mineral oil based. Mineral oils, having thicker oil films, are much better at lubricating roller bearing cranks. There would be no advantage to using a fully synthetic product in this application.
Following comments from Mobil's Technical Manager, our recommendation for the bikes you are stocking are as follows: Engine - Mobil 1 Racing 4T 15w50 fully Synthetic)
As you can see with the exception of Morris Oils, all who bothered to answer have recommended the use of Fully Synthetic oils. (interestingly at this time Morris did not make a fully synthetic oil)
I will continue to use and recommend the use of good quality fully synthetic oils using the highly shear stable VI improvers. This recommendation is not based on some hearsay, or myth. It is based on good working knowledge of oils, and Ural engines. During 2008/2009 I carried out extensive tests with the help of the Castrol research laboratory in the UK.
Urals were run on both a good quality mineral multigrade and Castrols fully synthetic RS 10w60. Samples where taken from the sump of these bikes at 1000 km intervals and tested by Castrol
Briefly these tests showed that the synthetic oil had not sheared at all and contained virtually no contamination of metals from bearings suggesting that the engine was adequately protected. The mineral oil showed considerable reduction in film strength even at 1000km and the mineral oil testing was abandoned at 2000km as the oil could no longer offer acceptable protection. The mineral oil also contained contamination from various metal compounds, presumably from the bearing or sliding surfaces of the engine.
I cannot recommend extending the oil change interval as standard practice even with these test results. Even the best oil in the world will still contain the acid by products of combustion and will get dirt. We have many customers who have extended the oil change interval using synthetic oils when it has been unavoidable during long distance trips. These bikes have suffered no apparent damage, but this is occasional rather than routine extension of oil change periods.
F2 Motorcycles Ltd make the following oil recommendations.
Engine 15w50, 20w50, 10w60, 15w60 fully synthetic oil.
Gearbox EP80, EP90. EP80w90, EP85w90
Final drive EP80w140, EP85w140
If cost is important and the engine is not being used under extreme conditions such as heavily loaded, towing trailers or off road the engine oil could be substituted for good quality semi synthetic of similar grades.
Mineral mono grades can also be used of the correct grade for the ambient temperature but they must contain detergent to carry the dirt to the filter. Non detergent mono grades are for vehicles with no oil filter as the dirt separate in the sump and drops to the bottom.
Never use mineral multigrades.