Oil is thinner when hot than cold fact! The oil grade is measured in two different ways, the first part for example 10w refers to the time a given amount of oil takes to drain through a given size hole, put simply a funnel. The second part, for example ‘40’ is measure using what can be described as a plastic pan with a bit of metal in the bottom, and an electro magnet on the underside of the pan, the current in the magnet is increased until it moves the metal fragment through the oil. The whole warm up thing is partly mechanical sympathy from old like a lot of things, the manufacturer says to use a 10w oil because at the given temp range that grade of oil will flow though all oil galleries at the correct oil pressure to avoid mechanical failure or premature wear, but when hot will not drop oil pressure to unsafe levels. And at the top of the scale the oil will remain the correct viscosity to lubricate and protect also That aside the minefield isn’t what grade of oil you use as such because you will see that grade changes with temp range of climate but more the devil is in the api ratings, additives, friction enhancers and such like and that’s where your manufacturer recommendations really count