For something to have worked fine before, and then only changing the oil and have it smoke I would have to suspect all of the suggestions given above. As to oil recomendation it has already been given, and just ake sure the API rating on the oil container has the grade of "SF" or higher (SG, SH, etc etc) you can always go higher but don;t go lower than recomended API service rating. The 10w40 in SF (or higher API service rating) is pretty common oil.
Here is something to consider.......and trust me folks are plenty slick when it comes to doctoring up engines to sell a problem piece of equipment or vehicle. "IF" this is the first time you changed oil since you bought the tractor, and now it burns oil and smokes even with the proper amount and API rated oil back in the crankcase, and the breather is ok, and you give it a little time for any in the muffler etc to burn out, and it still smlkes bad and uses oil, odds are the person that sold it too you never changed oil and it was thick as tar, or they used a higher viscosty grade of oil like say a 30 or 40 or even 50 weight oil or possibly doctored it up with a load of STP etc so it did not smoke until they could sell it. It happened to meon a VW rabbit diesel I bought one time used. It was winter and I ran it home from work, intending to change the oil. It was good and hot up to operating temp for quite awhile, pulled it in the shop, slid the drain pan under it, pulled the filter, removed oil pan drain plug, and went inside to eat supper. Came out about an hour later, and oil was still gurglling out of the oil pan drain. Finally it all was out, and I just attributed it to it being so cold outside even though the engine was hot when I started. I put new oil in the engine and a filter and fired it up, or I should say tried to fire it up. It wold not hit off no matter what I did. I was dumbfounded. I assumed I probably knocked a electrical connection or something loose during the oil change, but that proved false. What happened was the fellow I bought the car from had filled the crankcase with 90wt gear oil and #50 wt engine oil, so it would have sufficient compression to start. I took all that artifical compression ability away when I installed the proper viscosity oil. He did that to mask up a worn engine. Untill I was finally able to drive that beast out under its own power it cost me better than $1,800 more in money that I had to spend to rebuild the engine. Oh, I did refill the crankcase with the old oil as an experiment, and it fired right up..........so one never really knnows what another does to an engine so it seems like its in good shape.
Keep us informed I am sure all who have replied are interested in the outcome, now that y have the proper oil given and a source for the manual.
Regards