ahh old school doesn't work with modern oils
10w40 donates the "weight" of the oil at hot and cold. What it doesn't actually dictate is viscosity.
Old oils were ridiculously viscous when cold- the weight was the same but they flowed really badly. This meant you had trouble starting, stupid high oil pressure when cold, and actually very low flow- the pressure was high because the pump was struggling to push the oil round- at hot temps, old oils did exactly the opposite, got TOO viscous (or at least unstably so), and then it was very easy to cause starvation.
Modern oils in comparison are amazing. At cold fully synth should drop your oil pressure a huge amount. this is a VERY good thing. What you want when you first start your engine is to oil to get round the engine as fast as damn possible- with synth oil it flows so ridiculously well that you need barely an engine revolution to cover the whole engine in lovely protective oil.
When the engine is hot, the oils are also FAR more stable, yes they will go runny as designed, but they won't break down, and in "hot" parts of the engine- bottom of the piston skirt for example, they won't degrade, carbonise or give up.
On top of all this the soot dispersal properties of modern oils means they can have a hug amount more ash dissolved in them before they reduce effectiveness- their were some figures around a while ago, and the increase in ash and sulphur handling was more like 300% then 3%- i can't remember the figures but i remember thinking how impressive they were!
Of course what we buy as fully synth isn't really fully synth at all- what it is a fully engineered mineral oil- ie it starts as light crude and is cracked and refined to exactly the properties you want. You can get properly synthesised oil- used in race engines i believe- it is astronomically expensive- into £20 per litre for the proper fully ester stuff (like this
http://www.opieoils.co.uk/p-68900-fuchs ... ester.aspx ) which when you consider you might only use oil for one event and you might have 8litres with a dry sump system- you can see how it would add up!