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. 

Consequences of using wrong fuel

Wrong Gas fill up in Car

Q: What are the consequences when you inadvertently pumped diesel fuel into an automobile gasoline tank. I have been told this happens frequently.Any quick fix when these occurs?

A: The biggest problem I’ve seen is a headache. Why a headache? Because the driver of the vehicle doesn't happen to mention as I am going bats trying to diagnose a starting and rough running problem that they PUT THE WRONG FUEL IN!! Oh, "I didn’t think I put that much in so I didn’t think is was worth mentioning". In our region even the diesel fuel pumps have the same size nozzles as the unleaded fuel pumps.

But in real,yes this does happen. When it does happen, the best and quickest fix is to drain the gas tank. It doesn't take much diesel fuel to really screw up today’s computerized vehicle’s. Not saying anything is just going to cost you labor$ until you tell the whole true.

Adding Fuel Additive to Fuel System

IS IT GOOD TO ADD FUEL ADDITIVE

Can Fuel Additive Kill a Fuel System? Yes OR No?

Q: Actually I added some STP "Jet Fuel" fuel additive to my vehicles fuel system. And since after that my fuel pump goes out. Is this a problem with the additive or a coincidence?  PleaseHelp!!!!!!!!!!!!

A: Its not good to be applying mechanic tricks without mechanical knowledge.
Why did you add the fuel treatment? Did you feel that your vehicle wasn't running right? I often find that customers, who normally do not use fuel additives, will suddenly change their habits when their vehicle doesn't seem to be running right. More often than not, this is a reaction to a problem that is surfacing with a vehicle, but has not fully manifested itself. Fuel additives such as STP’s Jet Fuel as with other such additives are meant to be used as maintenance products. These additives are not meant to repair a part that is failing. Their job is to help clean and maintain engine performance. There is no doubt in my mind that your vehicle’s fuel pump was failing. It is just a coincidence and nothing more.