I use 4 and 5 most of the time. I will use 1-3 if I want something legato and using 3 will help this. I can't say I use 1-4 on black notes all the time, it depends heavily on where my hand has came from, and where it intends to go after the octave. I will change finger on the top note quite often when there is time, like play 1-5, then change to a 4, and then play the next octave with a 5 and then change again, or vice versa to get a passage legato.
I sometimes use 1 then 345 on top if I want a much bigger sound.
I know someone who can use 2-5 very easily, and can play that variation in Brahms Paganini variations Bk 2 with the octaves and single note alternation using 2-5 on the octaves.
Im sure in the video of Hamelin playing ala italia from the Busoni concerto he plays a passage of LH octaves using 2 5. The bit when the LH is above the RH and both hands are playing octaves. He has huge hands as well apparently.
I can remember when I couldn't play octaves with 1 4, and had to use 1 5 all the time. Luckily my hand got bigger!