I am looking for consumption-check and oil-refil

I think contracts and services are the same because you can have contracts with the type "service". And you also can have service with the type "contract". 

What is the intension of seperating this? 

I need :

• a contract Expiration date; just for warning purpose; Like for "every 6 month check oil"
  the maintenance module does not fit here.
• The Kilometer (Odometer) value at every service/contract. For consumption check.
• nice to have would be the price per unit with the units purchased.

gasoline bills can be managed in module:expense. (1) Is that the best way?

Reocuring services per car/equipment can managed in module:fleet. Especialy if there are more reocuring actions per equipment like general inspection, oil-check, wheels-check, etc.

(2) Is that ok to use the fleet module for maintenance?

thx. Rudi