WELCOME to LMF....
Almost all use a multi weight Synthetic Motor oil like Mobil1, but fortunately we have a Honda pro as a member here, He works for Honda and I'm sure he will correct me if Honda is different from the rest....
As for sales... eBay is always Great, CraigsList also, BUT around 6 mths. ago Muhammad said we were soon to have a Wanted/ForSale forum HERE.... (so untill it gets set up, Post it Here), pick your forum, either Honda or Small Engine & Mower Repair.... should work fine for an occasional, individual NoN pro parts sales.... :smile:KennyV