I have a Karcher model G 2401 HB and need to know what type of oil to use in pump ?

SOURCE: Can't fine where to put pump oil in

Dear glenoaks06,
I handled various Karcher washers, but the 2500 I never came across it. Just the same I will give you a few hints and maybe the solution for the problem. When the oil is milkey this means it has water in it. This means that there are faulty seals for sure. Now in many Power Washers the compartment where the oil is located is filled with oil prior to assembling the head unit that contains the pistons. So you have to take apart the the complete head change all seals( HP seals, LP seals and OIL seals). There are pumps that take multi-grade oil(15/50) and others 90 grade oil. If your pump is fan cooled, it probably takes 15/50 oil. Now one of the main biggest problem of water in oil is Faulty and Scratched PISTONS.
So when you have the pump apart....check condition of pistons. It is useless to change seals if pistons are damaged.
Now one thing regarding your problem, is that you have to put together the head with the pump upside down, so that you can fill the compartment (where the pistons are pushed by the swash plate), with OIL, before you put back the complete head with good pistons,springs,seals, valves...etc..etc. These few hints are relevant to quite a number of Karcher washers.
Regards,
Fulltech.

SOURCE: Karcher G3050OH Pressure Washer Spill / Unloader Valve (Bypass)

You should put 'locktite' on the male thread and screw it in all the way and lock it there. That is how it works ,the spring keeps the valve at the bottom closed during high pressure operation. When the gun is released the part with the o-ring is forced down resulting in the valve opening and then the pressure is by-passed.Those dont normally give trouble. Look for a blocked nozzle or a restriction somewhere in your high pressure hose.

SOURCE: model g 2401 hb

Very simple fix, I would recommend purchasing a replacement unloader valve, see this link, it should fix your issue