We have a 2018 GMC Acadia SLT with 3.6L engine used mainly for local driving on trips of 20 miles or less. We are not equipped for towing. Current mileage is 58,200 miles. I'm wanting to know if there is any requirement for replacing the transmission fluid and if so, when should it be done. In the owner's manual under the Maintenance Schedule for "Normal Service" it shows no service requirement for transmission fluid whereas under the schedule for "Severe Service" it recommends the fluid be replaced every 72,000 miles. However, the GMC dealer where we have the vehicle serviced says transmission fluid should be changed every 50,000 miles on a vehicle used for "Normal Service" but said we should be okay to wait til 60,000 miles. So which is correct? We're coming up on 60,000 miles and don't want to spend money unnecessarily but neither do I want to risk a transmission problem or void the power train warranty. Any thoughts on this?