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.
Should you swap out the fluid every few thousand miles, or wait until you've driven 30,000 miles or more? My owner's manual says I should change the transmission fluid every 30,000 miles, but my father swears it should be changed as often as engine oilevery 4000 to 5000 miles.