Oil viscosity selection is based on the lowest and highest anticipated ambient temperature during the period the oil will be in the crankcase.

Manufacturer's recommendations vary slightly, and in recent times they generally recommend lower viscosity oils than twenty years ago, but as a rule a 20W is okay to 20F, 10W to about 0F, and 5W for below zero, except arctic winter climates where 0W is the best choice. Using the logrithmic mean of 10W and 20W, 15W is okay down to 7.38765 degrees F. The lower viscosity range recommended by many OEs nowadays is an attempt to lower internal engine friction and eek a slightly higher number on the EPA mileage rating.

At the high end for summer temperatures the viscosity should be 30 or greater, but some manufacturers are recommending 5W-20 for year round use in all climates.

All owners manuals have recommended viscosities for expected temperatures, so if you want authoritative information for your specific make and model, consult the owners manual.

Duke