How to top up engine oil without making it worse
Most oil-related damage happens between services, not during them. Checking properly takes two minutes.
Check it cold and level
Oil drains back to the sump over several hours, so a reading taken minutes after switching off will be low. Check first thing, on level ground, with the car cold. If you must check warm, wait at least ten minutes after the engine stops.
Add in small amounts
The distance between minimum and maximum on a dipstick is usually around a litre. Add 250ml at a time, wait a minute, and check again. Overfilling is not harmless: the crank whips aerated oil into foam, which does not carry load, and the excess can be forced past seals.
Use the same specification
In an emergency, mixing two oils of the same grade and approval will not damage anything. Mixing specifications dilutes whatever protection the correct oil provided, so treat an off-spec top-up as a way of getting home rather than as a fill.
Investigate consumption
Some engines are documented as using up to a litre every 1,000 miles. Anything beyond that, or a sudden change in the rate, is a symptom rather than a maintenance task. Blue smoke on start-up points at valve seals, and a sweet smell in the coolant points somewhere more expensive.