That's a bit of a two-part myth there, at least partially so.
For one thing, the 75% quote is used to describe normal wear on an automobile engine, and 75% is a bit of a conditional exaggeration. Certainly, cold staring is a problem to be dealt with, but the wear that occurs during startup is not 3 times that which occurs in, for example, the top 8mm of the cylinder during high RPM operation under power in a typical MX four-stroke. Also, the oil has to have the ability to prevent failures under extreme conditions that would not be considered normal wear events.
Secondly, the cold start protection comes from the anti-wear additive package, which can be the same regardless of the base stock the oil is made from, and from using multi-grade oils such as 10w-40's that are thin enough at low temperatures to flow freely and quickly through the oiling system. Multi-grades are also available in both synthetic and natural petroleum based oils.
Synthetics, per se, are no better at cold start protection than petro oils unless you specify Group V esters, which do have the advantage of polar behavior. That is to say that the oil film has a tendency to be attracted to and cling onto metal parts to a greater degree than other oils, so they tend not to drain off completely over time. In other premium oils, the anti-wear additives pretty much take care of this without relying on polarity, but it's a nice feature, nevertheless.
Then why would you not use a group V oil?
I did read that Amsoil was originally made from a group V but now from a group IV. Wonder why they would change?