I've had both Shell Helix Ultra Racing 10W-60 and good ol' Agip Racing 10W-60.
The Shell was thinner and close to a 5W when when I poured it into the engine, the Agip is closer to 15W when cold.
The pour point is also much lower: -42C for the Shell, -33C for the Agip. The viscosities at 40C and 100C are however identical.
The engine starts up nicely at current temperatures with the Shell and you don't feel much difference between a hot and cold engine, as opposed to the Agip where the engine felt resistive when cold and below zero. And Opie Oils has it, too (albeit only the older API SM version and in expensive 1L bottles).
I reckon that the Shell and Mobil 1 (which is even thinner than the Shell, but also at operation temperature) are made using newer technologies, while Agip, Gulf Competition, Selenia and Miller are traditional full synthetics with ester and Castrol Edge being somewhere in between.
Last edited by WhateverBro; 22-01-15 at 22:33.