240;93728 wroteI used a synthetic 10w40 oil for my 240's last oil change simply because it's what I had, and there have been no detrimental effects. This is with approx 410,00km on the engine.
Only thing I've noticed is that it seems to burn oil more quickly than before (I'm having to top up every two weeks or so - previously, on 20W50 mineral oil, it was at least 6 weeks between top ups.)
That's because you've gone too Thin!
No issue going between synthetic and non though, you're thinking of diesel oil vs petrol car oil.