Anyone know if it would be an issue to mix different grade oils?

I believe the Sciroccos use 5w30 (correct me if I'm wrong). I was told that I can run a heavier grade oil and it would even whiten down the rattle of the engine. Recommendation was to run 5w40 or even 5w60. Any one done this?

So, here's the dilemma. My car is due for a service is a couple of thousand Km and I usually have a can of oil in the garage to ensure the oil is topped up. I've run out of 5w30 and didn't want to buy another 5w30 if I can run a heavier oil but if I can run a heavier oil, can I mix the 2 before the service and at the service, dump the old oil and replace with all heavier oil.

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