If you're going to be doing this on a regular basis, I recommend doing an oil analysis to make sure it's working out ok.
I plan on mixing 15w40 Schaeffer oil with 10w30 Schaeffer oil, however both use almost identical additive packages so it should not be a problem. My goal is to get an oil with a final viscosity of between 12 and 13cst at 100c, or a high 30wt to low 40wt oil. Once I see the first oil analysis results on it I can decide if I will continue running this mixture, or to tweak it one way or the other (adding more 10w30 or adding more 15w40 that is)