It might seem like an odd swap but my Ultegra BB is wearing out, I’m short of pennies and I have an SLX BB hanging about.
Any reason I can’t or shouldn’t replace one with the other?
–Twirlip

You won’t end up with the right clearance for your sprocket.
Better to get the right BB rather than mess up your chainset.

I’ve done it the other way. I know they’re spaced slightly differently but it was either bung the spare road BB cups into my MTB or not ride. So I put them in, adjusted the mech and made the ride. As it happens I left them in for a good few wweks without problem but did swap back to MTB BB cups once I got round to it.

I would be a bit more hesitant putting MTB stuff on my road bike just ‘cos the tolerance seem a bit tighter so it may[/b] not be possible to adjust for the different spacing.

To be honest, unless the Ultegra BB is really borked I keep it in ’til you can afford the right replacement.