To increase hot pressure with increased oil viscosity, we would want the second number of a multigrade to go higher.

The second number of a multigrade indicates higher temp (212F) viscosity.
The first number relates to viscosity below freezing temperatures

Single Grade (SAE30, 40 and so on) also indicates viscosity at 212F, but cold viscosity falls wherever it may.

So....to experiment with a thicker viscosity when hot, if currently using 10w40, you would generally want to go to a __w50.

15w50 and 20w50 are pretty commonly available and that's what I'd recommend trying next if you wanted to do that experiment.

5w50s are available if you're concerned about cold weather performance. I've never seen 10w50 but am aware 10w60 is a common grade
in some warm weather locations outside the US.