You can only use 10w40 if it is listed in your owners manual as an acceptable weight of oil for the engine.
not recommended
There should be no problem with mixing them.
SAE 10w40
900ml of4T oil 10w40
In those years..they used 10w40.
Sae 10w40.
10w40
The majority of 4 cylinders use a good 10W 40 Motor oil. No It Will Do Just Fine With 10W40
ATV engine oil 10W40
10W40 or 15W40 semi-synthethic
10W40 is a thinner oil, and would allow the engine to run more freely, and although both are suitable for winter usage, a thinner oil will usually help the engine start more easily in cold weather.