What could happen to your vehicle if you use the wrong engine oil, for instance: 15-40w instead of 20-50w?
Similar to your other question you need to us what the manufacturer specified. If you do not then you run the risk of the engine not performing as it should thus causing internal damage that you may not see right away but down the road you will for sure.
The oil types indicate how viscous it is, essentially how heavy and thick it flows. Put in something much heavier than the engine is designed for, and it will have trouble getting it where its needed, it can also reduce engine performance/efficiency slightly as the moving parts have just a bit more trouble powering through the increased resistance. Much lighter, and it won't stay where its needed, parts aren't sufficiently lubricated, and you start having wear and tear problems.
I should note that there are situations in which it actually is a good idea to shift to a different oil weight than is recommended by your manufacturer. Going slightly lighter in cold winter months for instance can help offset the effect the outside temperatures have on your engine fluids. That kind of shift isn't as necessary in modern vehicles than it used to be though.
Edited: daishain on 26th Mar, 2018 - 11:39am