I have used a mix of stuff in my push mowers - whatever I had on hand -
10w30, 5w30, 10w40, straight 30, 15w40 I doubt it matters.
If you want to play check this out.
http://www.nimacltd.co.uk/viscosity-calculators/calculate-the-blend-ratio-of-two-known-oils-to-achieve-the-desired-viscosity