B1 manual says 10w40... I have accidentally picked up a 5w40... Is it close enough or should I go change it for the 10w40!?

5W40 is fine for an engine provided you aren't trying to lube a gearbox with it. Regular gearbox oil is EP80 or EP90.

You may get away with it on very cold days, but I did this once with Mobil1 many moons ago on a CBX750 and the box 'stopped changing down' when it warmed up. I had to kill the engine and rock it through the gears to neutral to get going again. No damage done, but a hard lesson, and it took a few days for the thinner oil to properly disperse before the box started behaving itself.  You only need 10W40 semi synth anyway.

Go change it to the recommended oil...the end.

5W is thinner than 10w when it's cold and is will be fine until you come to change it again.