If it is smoking it is more than likely a blown head gasket. Common issue with this engine.
The oil to use for the 26hp Kohler motor on a Craftsman garden tractor will depend on the temperature outside. Use a 10-30 blend in the summer and a 5-30 blend in the winter months. The brand of oil does not matter as long as it is not a synthetic blend.
26hp
Locate oil drain , usually a small pipe with a cap on it, open it, remove the dip stick let drain, replace cap , fill to dip stick level,
It has 26hp
my 1998 Polaris Trailblazer has about 26hp and i have got it up to about 60mph
0.004 clearance both valves both sides
Who do you think knows more about what weight oil you should use? Your husband or the company that designed and manufactured your Kawasaki engine? It would take thousands of miles to see any damage done and by then it would be too late to do anything about it. Use only what Kawasaki recommends and nothing else.
It is hard to give you a definite answer without knowing the size of the engine, transmission type (automatic or manual and number of gears, and whether or not it has overdrive), and if it is 2 or 4 wheel drive. However, it is a truck. Trucks' aerodynamics are horrible, about twice as bad and sometimes three times as bad as a cars aerodynamics. At speeds above 35mph, aerodynamics play a very important part. The faster you go, the more important it becomes. I believe it takes around 13 horsepower (HP) for an average car to travel at 65MPH. With a truck that is twice as bad in aerodynamics as a car, it can take around 26HP and it can take quite a bit more fuel to make that 26HP. Driving techniques play just as big a part as aerodynamics. If you are interested in improving your fuel economy I would strongly suggest looking at www.ecomodder.com and www.gassavers.org