Stihl recommends 3/8 lo pro for the MS 241.
The bar and chain to switch the 261 were and even swap cost wise. He did end up having to take 2 drive links out of the chain but he didn't charge extra for it. He was going to swap the sprockets and I said I also wanted the .325 sprocket so he let me have it too.
The 241 is hair smaller than the 261 but I don't see any point in it. It is only $50 less than a 261. The 241 is also about 1 HP less.