Most minor, single mouth ulcers are caused by damage to the mouth, for example by accidentally biting the inside of your cheek while eating, or from a sharp tooth, food or filling.
I have a 2005 Honda Element. The owners manual states the brake fluid should be replaced every 3 years. When I asked the dealer to replace the brake fluid, they said it was going to cost $190. I told them last time they did it, it was only $27.
When you change transmission fluid may be more use dependent then motor oil. This is one item I would err on the side of caution. Though many change their transmission fluids every 30 k, at least with cars I own, halving their rates to 50 k or a little less for vehicles with heavier use seams reasonable.