I'm finally getting around to changing the oil in a 1914 Touring that I have owned for sometime. I know that the book says to keep the oil level between the two petcocks. Approximately how many quarts will I need to put in to achieve this level? Is it necessary to open the upper petcock and keep adding oil until some runs out?

Just put 4 quarts in it.

Keyword search is your friend:

http://www.mtfca.com/discus/messages/80257/99318.html

4 quarts will do you with perhaps one additional as a flush.