not all needles or threads are exactly the same. i am definitely not an expert on this subject. but my machine's manual recommends a size 14 needle when working with 40 weight thread. that size also seems to work best with my 50 weights. a size 11 needle should work well with 60 weight thread ... and most 50 weights.
with needles ... the higher the number, the bigger the needle - and the hole.
with threads, it's the opposite. the higher the weight number, the thinner the thread. (which makes no sense to me, but there it is. i mean ... c'mon ... when my weight goes up i certainly don't get thinner. go figure. :lol: )