Hi everybody! Just changed the oil on our 1 year old craftsman pushmower. It took 20 oz. We put Rotella 30 weight oil in it (because we got it on sale). The oil we drained was the factory origional, dirty as heck. Take a guess where the drain plug is..... It's under the mower by the blades! Was that a good oil choice, or should I use something else next time? Although the owners manuel says change it every year, should I change It twice because it only takes 20 oz?