Seeing as how this thread has gone into detail on oil, I’ll add some additional info that doesn’t get passed along very often.
On multi-weight oils, you’ll have an oil with basically two different temperature ratings. For a 10W-40 oil the ‘10W’ refers to the equivalent oil weight of this oil at low temperatures (ie. 32°F or lower) while the ‘40’ refers to the equivalent oil weight or grade at high temperatures (ie. 212°F). On a single weight oil, the weight or grade is rated only at 212°F but is assumed that the viscosity equivalent of the oil is the same grade at 32°F due to not having additives in it to alter the viscosity at various temperatures.
A 5W-30 oil will have better oil flow and startup characteristics than a 10W-30 oil due to the 5W-30 weight oil having a reduced viscosity at lower temperatures. This might be a player in the Northern States where the temperatures gets considerably cooler in the winter months but not as much as a player here in Texas on the older vehicles where it rarely gets to the freezing temperatures in my part of the country. For this reason, I typically always recommend a 10W-40 oil for year round use in the older engines as this provides a sufficient viscosity at the higher summer temperatures while also insuring adequate zinc/phosphorus levels even if the oil has the newest SM API rating.
The other item to remember on multi-weight oils is that the greater the difference between the low and high temperature ratings, then the greater the propensity for the oil to break down over time and not actually retain its rating. For example, a 5W-30 oil or a 10W-40 oil will break down quicker than a 10W-30 oil. But if the oils are changed out at regular intervals, then this breakdown issue is a non-event regardless of the grade being used.
Lorena, Texas (South of Waco)