Definitely check the oil, especially as you've changed it recently and swapped to a higher viscosity. Also check the clutch adjustment first.

However, if it's just third gear (you'd expect the oil/clutch to affect all gears), it could be the first signs of a worn stopper lever arm and/or shift shaft? This is relatively easy to check by removing the clutch cover and inspecting it for play. If it is worn then it can lead to more serious gearbox problems if not attended to. The parts can be replaced relatively easily with the engine still in the frame.

Read this thread from post 87.

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