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Here's the most honest answer you will ever get. I have no idea if it's okay to use 15w40 over 10w40.

I also, don't believe many could know for sure.

What I do know is your manufacturer said to use 10w40 and I would believe them, unless they subsequently amended that advice.

For what it's worth, it may be helpful to understand how multi-viscosity oils work. Initially, there was only straight weight oil, which would lose some of their viscosity as the oil heated. Therefore, they would suggest a higher viscosity, so it would remain effective as motor heated up during use. However, that meant less lubrication at and near startup and multi-vis was born to address this. They start at lower vis and it rises as the oil heats. How? With chemical additives. Essentially, you are buying 10 or 15 w oil that has an additive that will increase viscosity with heat. Sort of.

So, your engine manufacturer thought that 10w was the right to get the motor up and running. Will 15w do the job. I don't know. You decide.
 
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