Engine Oil

when hot one will behave like a 40 weight oil, the other a 30 weight. (bigger number means a little thicker effectively) low temp thickness should be the same.

if both are specified then you can use either, otherwise just use the one that is listed for your car.

Would there be any difference in for example mpg, performance, longevity by using the 40 over the 30 or vice versa?

My car is specified for the 5W and 10W 30's and 40's but I always put the Zeros in because I believe it to have a slight edge in the worst of winter when starting up from cold.

Slightly worse fuel consumption, longevity can go either way.

John gurgled happily, sounding much like they were saying:

Very unlikely.

Most unlikely, unless your battery or starter motor are shagged. More likely is that the thinner cold oil may cause the oil to drain back more readily and completely whilst parked, increasing start-up wear.

You bastard, I was going to say that.

Steve