Follow the manufacturers recommendations, it's in the owners manual.
If you do not have the manual, try looking at the oil fill cap. You may find it there.
2003 mercury 3.0 is a 5w20 it says so on the cap and manual.
Looking for a Used Sable in your area?
CarGurus has 562 nationwide Sable listings starting at $2,695.
Search Mercury Sable Questions
Mercury Sable Experts