On this engine, bad intake gaskets are far more common, versus bad head gaskets
And the intake gaskets very often leak externally, coolant, engine oil, or both
Exactly how did you determine "there's no water in the oil"
The best thing to do would be to drain all of the hot oil into a tub, so that you can get a good look
I wouldn't be at all surprised, if when you pull the intake, the gaskets are split wide open, or have crumbled into dust. If so, then it's quite likely the only problem
BTW . . . one thing kind of confuses me. You're talking about a "water leak"
We're all assuming you mean a coolant leak
Is that correct?
Before you tackle the intake and/or head, take a very good luck at the firewall area, with a good light and a mirror. Trust me, there ARE heater hoses going into the cab. Could be a leaky heater hose, or perhaps the heater core itself, meaning the pipes that poke through the firewall, into the engine bay
You might even have to put the truck on jack stands, and look from underneath