Straight SAE 40 is great for like stationary power in continuous operation, few startups, marine apps, although marine is going multigrade. In a vehicle it doesn't make alot of sense because of the way it's used.
It works well when the engine is operated at a constant speed for very long periods of time, the oil is at the proper temperature and becasue the base oil is close to the actual viscosity of the oil it lasts for a long time and is cheap. Multigrades can cause ring fouling in this particular instance because the additives used to create a multigrade can breakdown on the hot piston rings and over time build up causing fouling. This may not be true anymore.
The only time I ever heard of anyone using straight weight oils is a friend of mine who had his oil changed while on a trip to Mexico (SAE 40).
In your application the oil will spend alot of time below operating temperature and will be exposed to hundreds of startups. Under these conditions a multigrade oil is much better. A "thinner" oil such as 5W30, 5W40 will quickly arrive to the bearings and valvetrain on startup. This is very important to the longevity of the machine, greater than 50% of wear occurs during this period for the life of the vehicle! Also when the oil is cold the oil pump will bypass a certain volume of oil to keep the lube system within safe pressures. With a straight 40 weight oil enough volume may bypass that the volume of oil is insufficient to protect the motor if under load for the fist few minutes driving even though the pressure is correct!
I find it hard to believe that 'thin' 10W40 will be almost exactly the same viscosity as your SAE 40 at operating temp, but it's true. Multigrade engine oil has allowed engine manufacturers to build much more precise machines for the everyday consumer. Without it you couldn't build the highspeed, highpower automobile engines of today.
Hope this provides some rationale to the use of "thinner" multigrades. If the engine was rebuilt properly any premium quality multigrade engine oil should do the trick. These are popular grades for gasoline engines: 5w30, 10w30, 10w40. The actual grade should be called out in the owners manual, it is the correct grade so you can't really go wrong with it.
BTW save that bottle of Castrol SAE 40 'till it's winter, leave it outside overnight and give it a shake. Let me know what you think.