How quickly should oil pressure come up?

Most of what people said about oil is correct. SAE XXW is the viscosity measured at -20°C to -35°C depending on the grade. SAE 15@ is measured at -15°C and each lower W grade is 5°C lower in temperature. This is the lowest temperature at which the oil is guaranteed to allow starting and oil flow. Below that is risking oil starvation during a VERY cold start which seldom happens with our cars.
SAE 10W-30 and 10W-40 differ only in how much viscosity modifier (a polymer that thickens the oil only at high temperatures much like STP does). Otherwise they have the same base oil and additives. For older cars, I don't like using SAE XXW-20 as there is so little thin film lubrication with that low of viscosity. The SAE grade (30, 40, 50, etc) is measured at 100°C (212°F) so that is fairly hot.
Personally, I use Chevron Delo 400 SAE 15W-40 in all my older cars as it has the right level of ZnDTP to protect cams. Used it in my 426 Hemi, 383 Cude ('67), and 340 Cuda ('68) and never had any wear issues. For modern engines, I use Costco synthetic 0W and 5W-20 with great success. Due to the volatility of these low viscosity grades, I don't like using them in older cars that are not designed to use such light viscostiy and high volatility base oils.