If your car's owner's manual say the recommended viscosity is

10w30 or 10w40 ..

What happens if you use 5w30 or 5w40 ??

Is 5w30 too thin for a 20 yr old car ?? Corolla GLi 4afe ..

Currently using Mobil 10w40 ... Last year was using vTec LEV 10w30 ..