How do i fill oil on my bush jackhammer - Bosch Brute Breaker Hammer 11304

ROTELLA #40 oil or any 40 weight oil

The GREASE for this heavy jackhammer
Makita 181490-7 Grease (if there are short carbon brushes to find ) replace all orings gaskets (and looking for an maintenance kit for this machine) 188015-8 thats the better way to service.

have you checked the bushes

Is not enough this is a better suggestion for you because of wasting time for only oil (lubrication is not only very importand because of heating but if it was leaking theres more to do 323745 service kit comes complete with the most neccesary parts allways but priority in this is extra (in most cases) change the strokebolt(second hammer) allways by if that looks not smooth surface.
https://devs.hitachi-koki.co.jp/ois/explodedDiagram.html?type=svg&modelName=H+65SB2&newwindow=true

Replace O-Rings?

I’m happy to help further over the phone at https://www.6ya.com/expert/jason_60e0f57ba319f3f6

First of all you must locate the oil leak if is from top rocker covers, timing side crank front oil seal,rear crank shaft oil seal? Is it from the oil filter?,or is it from the front lower wishbones inner bush. That bush is filled with hydraulic fluid,and most probably that is your oil leak..If is from that bush the thing you do is just to renew the wishbone.

how much oil do i put in my bosch jackhammer?

check the oil in it

usally their is grease packed in the hammer part,you can usally see a plastic type bung(plug)on outter casing

# Balance the load
# Level the machine
# Inspect the water pump for a blockage
# Inspect the coupling
# Inspect the transmission
# Adjust the drive belt tension
# Check the motor bearings
# Check the drum brakes