petrol and engine oil are mixing

The petrol and engine oil are mixing, what could be the cause of this?

fuel is mixing whit the oil already change the fuel pump

Your problem is going to be an injector(s) or injector O-rings. Fuel can leak out of the solenoid of the injector into the oil, or the injector o-rings can leak into the oil also. 

You will need to bypass the fuel filter, add some clean fuel in a bucket, add some fuel/oil dye to the fuel, and run the truck off of the bucket of fuel. Take the return off of the back of the head, and also run this into the same bucket, let it run for a little while off of this fuel, remove the valve cover, jake housings(if equipped) and rocker arms, take a black light, the dye will glow where the fuel is coming from.

I would not rule out an injector, I have seen it more than once, but the most common is injector O-rings.

You should have 3 o-rings, they should be pretty close to the same size. It is also recommended that you replace the hold-down bolts because they do stretch. 

Detroit came out with an Auxiliary seal O-ring. It is very thick, and also you have a graphite seal that goes on the tip of the injector the accompany this o-ring as well. This o-ring is needed if you are getting fuel into the coolant due to a leaking injector cup. This is a quick, cheaper alternative than have to remove the cylinder head to replace the injector cups. If your injectors have these, you will need to replace these too otherwise you will eventually get fuel back into your coolant.