i currently use regular gas in my tank on my volvo s40 but i wanna switch the type i use and i was wondering if anybody knows what kind to use in my tank for it to run smoother and better

 

check the label inside the filler door - it should read "91 octane recommended" (don't have my car here so I can't check for you).

In car speak, recommended is different from required... which implies you are at risk if you run lower octane than required. Your car is a "low pressure turbo" so it will run on regular albeit full power will be cut back when the car detects any pre-detonation. If you are a sporty driver then you'd probably want to stick with the 91. That said, name brand premium fuels do contain more addititives which will keep injectors clean etc so even if you decided to run regular, its smart to throw in a tankful or two of name brand 93 to keep stuff clean.