Can I bulk muesli out with porridge oats?

[FusionChefGeoff]

Or will that give me digestive issues???

It won't give you digestive issues, the substance is the same but porridge oats are much smaller than the oats used in most muesli, which might meant hat the oats that were originally part of the muesli mix absorb the milk at a different speed to the oats you add, leading to a different texture. Jumbo oats would be better, or you can mix your own muesli from scratch and control the ratios of everything.

Very likely.
You need to either soak or toast them first.

Thanks I may try a couple of small experiment bowls rather than chucking the whole lot in!!

I often have both together but it's more porridge with a bit of muesli than vice versa. I would either soak the oats overnight, or make porridge as normal and sprinkle muesli on top.

I add them raw with no side effects so give it a go with a small amount and see how you go.

My husband routinely adds ordinary porridge oats to muesli to extend it, with no digestive issues that I'm aware of. He also adds extra nuts and dried fruit. Why he doesn't just mix his own muesli I really don't know, but apparently this is the best way.

Absolutely,

It's what muesli is, oats with some fruit and nuts added.