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.
How often do you really need to change motor oil? Do you change your oil every 3,000 miles? The majority of drivers on the road have been bombarded by advertising that recommends changing their car's oil every 3,000 miles, but the truth is that interval is no longer really necessary.