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: Most manufacturers recommend that manual transmission fluid be changed every 30,000 to 60,000 miles. Under heavy-duty use, some manufacturers suggest changing transmission fluid every 15,000 miles. Automatic: Service intervals for an automatic transmission vary from every 30,000 miles ... to never. The typical service interval is 60,000 to 100,000 miles. Changing it more often does no harm. Manual: In a manual transmission, the problem is not so much the fluid degradation, but rather fluid contamination.