Yes you can but find out what weight the manufacturer recommends first. Some recommend a straight weight instead of a multi weight oil.
4 stroke
No
yes you can use 10w40 oil in most dirt bike transmission but it might make your clutch stick on and it will be like your clutch is on its way out so try it and see if the clutch seems ok then happy days but if it don't go for the 10w 30 cos that's what most companys would recommend that you use in a 2 stroke engine
32:1
Soy
Your bike is a 2 stroke. since you are mixing gas and oil together oil naturally blows out your tail pipe. if this is happening, then there is nothing wrong with your bike, unless it is a 4 stroke.
If your 2 stroke dirt bike starts to smoke really bad that means you have to much oil in the fuel or the wrong kind of oil. Or the pistons, oil rings or cylinders are badly worn.
In the crankcase your looking for 10W40 or any gear case oil from you local 2 stroke motorcycle store. I use 80W Racing oil. If your talking about the fuel then you need 2 stroke mix oil with your gas and that ration I would suggest would be 32:1
Best to use two stroke oil. Mix it with the petrol, (I guess you know this)
standard 2 stroke oil snowmobile or dirt bike oil
Depends on the manufacturer as to what type and weight oil to use.
0.75 litres