5W-40 syntetich Engine oil 89 Sub 5,7 TBI

Hello, I'm a happy owner of a 89 Suburban K2500 with 5,7 TBI.
I've had some chevrolets over the past years, and have a question for you.
What kind of engine oil are you using?
I have used syntetich 5w-40 on my 350's, but the user manual says that f.eksampel 10w-40 should not be used. Anyone who do have a good answer for this? i might bellive its becaus its 10w-30/5w-30 gm has approoved their emmision with??? I dont see at there sould bee any problems using this 10w/40 oil either. (sorry for bad grammar :rolleyes: )
Best regaard Eyvind from Norway

A good rule of thumb is to use what the manufacturer recomends but I think you will have no problems running 5w-40. I use 5W-30 Mobil 1 in both my vehicles. Stick with synthetic though, it is better.