"Monograde oils are an acceptable option if the equipment can provide excellent oil cooling and constant temperature control".
OPE is not famous for either of the above. Monograde oils thin out at high temperature and thicken at low temp. The difference between them and multigrade is the steepness of the line. I see the primary advantage of a monograde is the missing viscosity improvers. Keep in mind that modern additives are better today than decades ago,
and synthetic base stocks (PAO and Esters) have a native high viscosity index to begin with.
Synthetic oils do not have a shear down or thermal / oxidation stability issue. Monograde oil solves a problem with multi-grade mineral oils, and this problem simply does not exist with high quality multigrade synthetics.
I fail to see a reason to prefer monograde over a high quality synthetic multigrade, but perhaps I am missing something. I am curious to hear rational argument to the contrary.
Take a look at this article if interested: