You can mix oil weights without causing damage. You can also mix brands, synthetic with regular.
When you mix 2 oils of different weights you end up with something in the middle.
Mixing brands is not recommended by auto manufacturers but they do not give a valid reason so is probably a marketing issue.
Video:
Oil Rating and Specs:
For example: 10w-40
10 is viscosity cold
w is for winter
40 is viscosity hot
Higher numbers are more viscous
It is bad to let your engine oil get too low or not change it every 5k miles.
