Mixing oils...

Hi:

I've noticed that the oil level has gone down on my bike. I've thought this could happen because last time I changed the oil and filter (some 200 miles ago) I put synthetic 15/50 instead of synthetic 20/50 (as I used to). Can I refill with 20/50 mixing it with the 15/50 still in the tank?

Thanks!!

Its ok to mix grades as long as the new oil is also fully synthetic

Thanks for your help!