I need to know if I really need to put premium into my tank or can I use a mid grade? I have been told both thanks!
There should be a label on the door or in the owners manual. If it says to use a certain octane, then use that. Otherwise use the regular gas. Answer = follower the manufacturer recommendation, there is no need to exceed it. Higher octane does not make your engine cleaner or more efficient. All it does is reduce pinging and knocking.
Yes, yes you do. It’s not a big deal though as Bimmers get better gas mileage with premium-nothing massive but it offsets the cost of the premium. On top of that the car will perform better with a more aggressive ignition curve.
In addition to running premium I’d suggest a top-tier gas brand or at least a respectable name in gas. Your clean valves and injectors will thank you. Oh, and I used to work for BMW if that makes you feel better.
In an M3 you definitely need premium. You can confirm if you like with your owner’s manual.
You absolutely need to use premium. The M3 is a high performance machine. Also keep in mind that if you have a E46, then you need to be very careful with what oil you use. IIRC the M3 takes a specific weight of oil that is heavier than most cars use. Awesome car though.
Castrol 10W-60 -available only at the dealer.
I agree with the previous replies. Compared to the cost to buy and maintain an M3, the difference in price between premium and regular is chicken feed.
The best way to minimize overall operating costs is to spend what you must to take good care of your car. When things break, get them fixed even if they are expensive. Letting repairs slide means that the car deteriorates. It becomes progressively more unpleasant to drive and restoring it becomes a daunting expense. Unless you enjoy detailing the car, take it to a professional every few months so that it continues to look good.
Thank you very much!
Thank you so much!
Thank you so much!
Thank you so much!
Thank you everyone 
You’re welcome