I'll try,  back in the olden days, shock companies & manufacturers of cars & especially bikes would put cooling fins on their shocks & reservoirs, so I guess it has long been the considered opinion that it is better to keep the oil cool, it seems only natural as hot oil is thinner than cold. That said you generally don't see all the fins these days, & mounting reservoirs in an air flow position must help. I can only assume that with all the new types of synthetic oils available that the heat is no longer quite as critical but like any oil it does deteriorate & is best changed after a certain amount of use. Pull an old shock apart sometime & smell the crap that comes out.