I have an older model front tine Wizard Tiller that hasn't been used in about 15 years. I have tuned it up and it runs great. But I'm not sure how to change the gear oil in it or what oil to use. On the top of the gear case the is a half inch plug and on the front there is a quarter inch plug. I assume that on is a drain and the other is the fill. My question is how much gear oil to use and what kind?