One of the main reasons main bearing failure seems much more common nowadays than it was maybe 20 years ago, is I think due to modern Efuels, and the almost universal use of dual purpose 2T oils, which are also suited to autolube use. Efuels in themselves absorb water, and where bikes using Efuels are stored for any length of time, there may well be problems with carb jets and drillings getting blocked, and in severe cases leading to the need for a new carb. Efuels also tend to degrade nitrile crank seals much quicker, and its a good idea to fit viton seals if these are available when it comes to rebuilding a motor. The dual purpose 2T oils are necessarily very thin so they can be used in autolube systems, which is less than ideal for pre-mix bikes, as the oils are thinned with kerosene or white spirit at up to 30% by volume, that means mix ratios must be increased to provide the same lubrication performance and corrosion protection as a pre-mix only full synthetic competition oil. However many users mix the dual purpose oils at the same ratio which would be appropriate for a proper pre-mix only product. As trials motors are very low stress, this isnt something thats likely to compromise lubrication. However due to the very thin dual purpose oils not providing as good film coverage as much lower viscosity pre mix products, to protect engine internals when bikes are not being used, this in combination with cleaning methods which often involve some amount of water finding its way into the motor, and you have a recipe for increased chances of corrosion, which will often result in early main bearing failure. The way to ensure the best possible main bearing life is to use a full synthetic pre-mix only oil, such as Castrol XR77 or Motul M800, mixed at 70:1 for air cooled and 80-100:1 for water cooled, and be careful to prevent water getting into motors when bikes are cleaned, and always run motors for at least 10 minutes after cleaning has been carried out. Replacing main bearings isnt an expensive job, but it is something that can be avoided in many cases, if the simple advice given above is followed.