Classic grade mineral oils were introduced in the early 1960s and solved the problem of using a different grade in summer versus winter.
Multigrade oils consist of relatively light base oils with the addition of viscosity improvers (VIs) to produce a high viscosity reading at 100 degrees centigrade. VIs consist of long chain polymeric materials that coil up tightly at cold temperatures and uncoil (and take up more volume) at high temperatures thereby increasing the viscosity of the base oil. This gives the oil the characteristic of a thin oil at low temperatures and a thicker oil at high temperatures.
Millers Classic Pistoneeze 20w50 (formerly Classic 20w50) is an all mineral high quality SAE 20w50 oil with a modern additive package to ensure that it stays in grade even under extreme conditions.