Different types of lubrications are utilized in transmission systems to lubricate tribological contacts in order to minimize churning loss and dissipate frictional heat. This project concerns optimization of jet and splash lubrications in truck transmission system to rise efficiency and lubricant in meshing areas using particle-based computational fluid dynamics (CFD). Because of insufficient oil impingement in meshing gears, besides oil immersion additional railways on top gears, inside input and shifting shafts are provided which in some cases can act as jets. This novelty can meet expectation to deliver acceptable amounts of lubricants to critical places in transmission system. In order to study lubrication system elaborately, shafts are analyzed separately and modifications to existing designs for casing, spline, radial channel and jet angle are carried out to ameliorate cooling process with respect to high efficiency.