If you don’t put anything on it you won’t get any benefit from it really. Do as you say and put a few tanks of gas on it each month. I’ve got a few store cards that I put one thing on then pay the minimum each month until they are paid, then do it again. Nothing major, just stuff like a new microwave.
The only problem is that if you don’t pay on time your score will go down. You need to use the credit cards for your score to go up. The card has to be used but not maxed out. Most people think maxed out is when the balance is 100% of the limit. The FICO score sees the card as maxed out when the balance is over 30% of the limit. The other thing that helps the score go up is the age of the account. If you have any old credit cards, don’t close the accounts. That will push your score lower also. To make the score go up get the old credit cards to raise the limits but don’t add more to the balance.
If you want to really raise your score, try to find somebody you know very well and trust very well (I used my parents) with an old credit card with great history on it. My parents have one with a HUGE line on it they haven’t used in years. My score went up close to 100 points overnight when they added me as an authorized user.