I don't know if Ive understood your question but for example the oil viscosity in Internal combustion engines decides in the efficiency because the losses during the lubrication process are related to the motor efficiency.

It makes a big difference if you're talking about cylinders, rods, and crankshafts in an internal combustion engine or bearings in a gas turbine (including single and dual shaft). Consider, for instance, why Wankel engines have fallen out of use... The swept area (contact per stroke) is far larger than for a reciprocating engine of the same hp. Not only must you keep it all lubricated, but each time the rotor turns in a Wankel, the seals sluff off oil, which greatly increases the emissions (requiring a large catalytic converter in the Maza models). The larger swept area also increases the heat loss (because you must cool the surface to protect the oil and preserve clearances), which is all wasted (requiring a larger radiator in the Mazda models). Any advances (teflon, ring coatings, lubricants) that might help the Wankel can be applied to the reciprocating engine, maintaining the gap, so that the Wankel engine is always behind. It's an interesting design, but not practical. Miracle lubricants won't equalize the disparity between the two designs. For most types of engines, lubricant factors are small compared to other efficiency losses, which is why efforts are more often focused elsewhere. If you're developing lubricants, there are other things to consider as well, including: heat breakdown, tolerance of debris, etc. Automatic transmission fluid is a great lubricant, but don't put it in your crankcase, because it breaks down with blow by (leakage around the pistons into the crankcase). Motor oil would work in an automatic transmission, but not well or for long because of the clutches plus it's too viscous.

Dear Malik N. Hawas ,

Please tell us more in detail about your question. DO you mean a motor as an engine?