Thicker/Thinner oils can have quite a different on oil pressure, and the associated (expensive!) parts that rely on it!
Personally I wouldn't worry about it too much. Even the correct grade golden brown engine all that is dumped in during an oil change is quickly contaminated by sludge and ghunk that simply removing the drain plug for a few minutes/hours won't remove, so I find it hard to get too worked up over getting the 'right oil in there for a top up. If you know what oil is already in the sump than brilliant, use that, but if you don't 10W40 is a pretty safe bet.