The only factory recommendations for mono grade that I have seen were for air-cooled engines like Briggs and Stratton (like most people here I always used 30 grade for 4C to 40C) but I just bought a new mower and even they now recommend a 10W-30 synthetic with mono 30 grade listed as "an acceptable substitute". Air-cooled aircraft engines used to use mono grade but I don't know if that is still true. I believe that the VI improvers in multigrade have themselves been improved over time and synthetics need less or no VI improver.