So originally there were single viscosity oils like 30W, etc. These oils were a certain viscosity (thickness) at a certain temperature and the viscosity varied directly with temperature, thicker when cold, thinner when hot.
Then came multi viscosity oils like 10-40W, etc. that attempt to maintain a constant viscosity throughout a temperature range by acting (for example) like a 10W when cold and a 40W when hot. So my question is what median viscosity is 10-40W oil trying to maintain? For example if I substitute 5-40W for 10-40W is it thinner across the entire temperature range or will it be the same but only thinner at cold temperatures?
Then came multi viscosity oils like 10-40W, etc. that attempt to maintain a constant viscosity throughout a temperature range by acting (for example) like a 10W when cold and a 40W when hot. So my question is what median viscosity is 10-40W oil trying to maintain? For example if I substitute 5-40W for 10-40W is it thinner across the entire temperature range or will it be the same but only thinner at cold temperatures?