Engine Oil 10w-40 or 5w-40?

[steve1001]

Gentlemen

Ecp and the like seem to be recommending 5W-40 for my 306 (X reg 2.0 HDI).

For the last 15 years I have used 10W-40 (B3).

I see the the 5W-40 is thinner in Winter for easier starting.

Will I do any harm using 5W-40, or should I stick to 10W-40, as per the Haynes manual?

Thanks

Steve