What should the oil viscosity be in a 1997 Jeep Cherokee 4.0L?

As thin as possible and still maintain oil pressure.

Answer 2

I'm pretty sure that 10w30 is the recommended viscosity, unless its really cold (below 32ish) then 5w30 is recommended.

I know some people like thicker oil and will use 10w40. Personally I believe that using 10w30 changed at reasonable intervals will make that engine last a really long time.