Well, it's fundamentally incorrect, but I can't say if he was blowing smoke, or if you don't remember it right. The one thing about it that is correct is that single weight oils don't usually have this problem. Multi-grades do not "change viscosities". They are made by taking a light base oil and modifying it with Viscosity Index Improvers (VII's) so that they do not thin out with heat as much as they ordinarily would. For instance, a 10 wt oil is modified so that it thins out so little that at 212 degrees F, it is still as thick as a straight 40 wt would be at the same temperature. So, rather than "changing viscosities, the does quite the opposite, and changes very little at all when heated. Straight grade oils rarely have any of these additives in them, and they can't be beat out of commission if they aren't there. The VII additives are big, bulky molecules, and most of the less expensive ones used in engine oils are physically somewhat fragile. Transmissions tear them apart, and they can no longer function as they should. VII additives tough enough for use in gear lubes do exist, which is where stuff like 75w-90 GL-1 comes from, but these additives cost more, and since they are not usually required in an engine oil, they aren't used. The oil in your bike is not just engine oil, however. Any contaminants that would change the viscosity (that I can think of) would do so across the board, at both ends of the temperature range, so I would say such things would also affect a straight grade.