Salty dogs like us know a "thicker" oil actually conducts heat better than thinner oil. That's why anything will cool faster in 70 deg water than 70 degree air, nothing else considered.
But it's not that simple. You need proper flow, cooling fins with air flow across them, etc., etc. 10-50 or any multi-grade is designed to maintain it's room temperature viscosity in the engine even.... as it heats up. The low number (not including the (0-20) is determined by how cold it is where you need to start your car.
Magnets: What are your thoughts on magnets on the side of oil filters or the type that screw on before the filter, and then the filter screws on to the magnet collar?
Kind regards, Harvey