Wa Alaikum Salam. Brother unless / until you do not mention the complete details; that which engine oils, their grades / viscosities had been used, or after how much interval oils their grades have been changed, all will be clueless and unable to figure out the problems.
May be, the oils were fake, may be, unsuitable / non-recommended engine oil grades / viscoties have been deployed, etc. but unless complete details are available, members will not be able to guide.