Multi weight engine oil has viscosity index improvers that reduce the thinning with increase in operating temperature.
An SAE 10-30 wt oil is base 10 wt with viscosity index (VI) improver (heat sensitive additives)
The VI improver additives are susceptable to shearing damage in non synthetic oil. The damage can occur very early in the life of the oil, reducing the effective weight of the oil from 10W-30 to 10W-20.
Ever notice that when the oil is changed when using non synthetic multiple weight oil, the oil pressure gauge shows an increase in pressure? After the oil is used for a while, the oil pressure gauge shows a reduction in the oil pressure because the VI improvers have been damaged by internal engine shearing action.
Using synthetic muti weight oil the shear damage issue is basically eliminated.
Still, Prefer using straight SAE 30 non synthetic and change it once a year in my N whether it is used or not.