Too Little Maintenance- Can lead to breakdowns and costly repairs, some of which may leave an older vehicle not worth fixing if the cost of the repair is higher than the value of the vehicle (like an engine or transmission replacement).
Too Much Maintenance("up-selling")- Rarely hurts the vehicle, but can be expensive and take funds away from more critical or unexpected repairs... like having an alternator or starter replaced.
("Up-selling" is when unnecessary services or items are sold during routine maintenance . These services and/or items usually provide little to no benefit to the vehicle, add cost, and only really benefit the person selling or performing the "up-sold" work)
To protect your vehicle from receiving too little maintenance, as well as protect yourself from being "up-sold" unnecessary services, its best to become familiar with your vehicle's specific "maintenance schedule" which can be found in your vehicle's owners manual. Becoming familiar with your owners manual can help in many ways. It holds a wealth of information.