I learnt to type when I learnt to program as a kid so I never add more than one space. Quite often an extra space can be the one thing that causes a program to eat itself and believe me, there is *nothing* worse than spending hours combing through code to find that you double-spaced in one line. It's so hard to find. Plus, now that fonts are spaced, there's no point - it disrupts the reader's view. (I have this discussion with my flatmate when I edit her essays.)

I use semi-colons occasionally. It's not something I remember learning in school but I do know the usage rules. I probably do use Oxford commas but not because I was taught to use them. I'm just a big coma user.

I won't start a sentence with 'but' but will use 'and'. Just never in a professional/academic situation.