Yesterday I checked my oil dip stick and for my surprise it was completely dry, I checked the stick a few times but again, dry! So I ran to the store and bought 5qt of oil and put 3 1/2qt and when I checked again it was way over the full reading mark. Does anybody knows what could happened here?

 

I don't have your generation MINI, but the low point on the dipstick and the max point is probably a liter or so if I had to guess. You way overfilled the engine.

 

Typically the low-high indicates on about 1 quart of oil. Should do a proper oil change right away to get the right amount. If you ever have to do this again, however, if it's not showing on the dip stick only go 1 quart at a time. Sounds like you're burning/leaking oil though so might want to look into that.

 

in other words

when you see 'nothing' on the dip stick it does not mean you are dry cuz the dip stick does not touch the bottom of the pan ...

it means you are at least 1 liter low ...... so add a liter and check again .....