JD z425 w/ B&S engine white smoke issue

Just took our JD in to the shop 4 months ago to have a throttle valve replaced to the tune of $600 when all said and done. Recently, this mower started blowing white smoke from both the engine and muffler after running for awhile. It starts out a little at first, then gradually smokes more....no noticeable engine loss or problems other than the smoke. When I first noticed it, I could see oil slowly dripping on the muffler, causing the smoking there. There is dirt/oil buildup between the muffler and engine block, but when I cleaned it off and ran again, looking for leaks, I didn't see any, not even on the muffler like before. I plan on changing oil, fuel/fuel filter, and spark plugs first, but does anyone else have experience or suggestions that a moderate diy'er can do? Trying to keep it out of the shop again if possible....Thank you in advance

JD z425 w/ B&S engine white smoke issue