3.5 litres is enough - it will show perfectly between the marks of the dipstick. Also for oil changes find some M12 copper washers 2 are enough, anneal them for each use. Dont throw them away.
to anneal - get a welder to heat them up to an orange glow and let it cool down - it will become soft again perfect for the next use. The reason I said 2 was that you can install a ready one and get the other annealed later. For sure your sump bolt will never leak. Just dont overtighten it like a gorilla would.