Theoretically , a multi grade with a wide viscosity rating should be avoided in engines with roller bearing bottom ends. The reason being that it is generally viscosity improvers that are used in such oils to achieve their viscosity spread , but these are sheared down more quickly in roller bearing type motors so the oil reverts to it's base lower viscosity , so in the case of say a 15w50 , it would become a 15 grade oil !! Not good !! Thats why mono grades are advisable. Things are not quite that simple though , if the oil is changed quite frequently by modern standards , ie 1000 - 2000 miles , a high quality wide viscosity range oil will probably not shear down by any significant amount , and , they are preferable in the respect of better cold start protection due to their lower cold temp viscosity. One way to determine if a chosen lubricant is performing ok is to have an oil analysis performed on a small sample. The resulting report will indicate any problem issues , such as larger than acceptable metal deposits which can indicate bearing problems for example , fuel dilution , etc etc . This is a normal routine for commercials and plant engines where engines are very expensive to maintain and overhaul , so there are a number of labs providing this service at reasonable cost.