Engine Oil and Low Viscosity

What is the application?

SAE 40 is a single-viscosity oil. Without knowing the application and environment conditions (but assuming you're in an automotive sectino), could you try a multi-grade one, like 20W50 or similar? Maybe you can locate some good one in a gas station or accessories shop near you. Single grade oils are not used anymore for a while... like 20 years or so. BTW, not sure if you're familiar, but 50 states for oil behaviour at low temp. 20 at high temp. (meaning it behaves like a 50 one in low temp, and a 20 at high temp).

I think you have that backward 20 at low temp, 50 at high temp.

If your code 154 is in fact 0-154 this is a NATO designation for helicopter transmission lube oil, probably not the best choice for an ICE of any type.

If you viscosity is in fact depreciating in such a short run time consider incompatibility of the oil used or look for some major internal fuel leakage. Again going back to the possibility of the wrong oil for the application I would speculate this stuff has a very high detergent package and it is just really cleaning out the insides or if you are suffering from fuel dilution you are getting the same results. No matter what if does not sound good for the long term life of your equipment.