I've got a 3 year old B@S 4HP push mower, I just changed the oil recently, cleaned the filter, and replaced the plug, the problem is this was the 1st time any of this has been done to the mower...should I change the oil again soon or wait till the end of the season? The new oil still appears clean, I did drain it out the bottom even though the manufacturer's prefered method is to turn it upside down. It runs good, not much smoke at all.