Don't put in more oil than is required. Otherwise you will have unnecessary pressure and oil will be spit out into the airbox.
After a change of oil and oil filter you need to run the engine for a few minutes (and then wait for a couple of minutes to allow the oil to settle) before you check the sight glass. This for two reasons: the oil needs to be warm ( as it expands when warm) and also because the new oil filter will 'consume' some oil.
Another point: see that the oil you are using is suitable for the ambient temperature you are in. In a cold country, 10W-30 oil would be much better than, for example, 20W-50 oil