Ideally, you'd only want enough feed to make enough heat to warm the required radiation. Short cycling isn't as good as long cycles. If your required heating needs cause the stoker to run for moderate periods of time, I would leave the setting as it is. If it heats up rapidly, you're eventually run into the high limit shutdown problems in warmer weather. You should keep a 40 or even 50 degree spread between the high and low limit on your aquastat.
The standard advice is to find out a good feed rate and leave it there all year round. 5 teeth is a pretty common setting; some can get by at 4 teeth. I very seldom hear about people higher than 5, but I'm sure they are out there.
Our salesman, with about 40 years in the service of stokers (and oil) says he never checks for efficiency. You feed coal as your general needs require, look for a good ash ring, keep it clean and lubed, and forget about it.
Your best bet is to start at a low teeth setting and increase one tooth at a time until your needs are met. Then, you may be able to stay at that setting all year long.