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.
Manual transmission fluid should be changed every 30,000 to 60,000 miles. If you change your transmission fluid yourself, be sure to use the correct type of fluid. There are different kinds.