My friends recommend using Peugeot 5w-30 oil. However, the car is 17-18 years old. It's leaking a bit from the engine's connection points, such as the gasket line. Last month, I had to replace the brand new one because it was leaking from the crankshaft seal behind the flywheel. I used 10w-40 oil in previous years. This year, we used 5w-30, and oil leaks increased. Which oil do you use? Should I use 10w-40 for a 17-18 year old car? Or should I use 5w-30 no matter what? The engine is clean, and it would be foolish to open up the gasket just because it's leaking.