I recently bought a 220 cc motorcycle.Manufacturer recommends 20W50 engine oil.I have seen owners manual of 250 cc motorcycle by another manufacturer where recommended oil viscosity is 10W30.I want to know what parameters are used by makers to provide a recommendations for engine oil and why is there a difference by two manufacturers for almost the same engine displacement
Like with AC compressors. Basically a matter of materials chosen for the internal mechanics, oil galleries design and internal clearances. The different combinations of these call for a different oil film thickness and hence different oil viscosities across a temperature range. Same or nearly the same displacement doesn't mean anything because every engine has its own engineering design behind it.