First, go get a couple more gallons of hydraulic fluid - if I recall correctly, my 304 holds almost seven gallons with the loader. Could be wrong, too.
I'd recommend running it a while and exercising all the hydraulics to get the fluid heated up before you drain it. Then shut everything down, dropping the loader arms and curling the bucket to collapse all the cylinders while the engine is running. Make sure the TPH arms are all the way down, too. Then pull the drain plug from the rear bottom of the hydraulic sump and drain all the fluid. Once drained, I recommend filling it with about five gallons of kerosene or diesel fuel and cycling everything again to flush any crud loose - about ten minutes should do it. Drain the kero and pull the filter. Fill the filter with fresh hydraulic fluid and replace it on the tractor. Fill the sump until the fluid just reads on the dipstick when the dipstick/fill cap is resting on the hole, NOT screwed in. Cycle all the controls several times until the whole system is filled and check the sump level and add fluid as necessary.
I'd recommend doing the same procedure with the engine oil. In that case, just let it idle for five minutes with the kero and then drain. Refill with 15w-40 diesel rated oil. Drain the oil from the fuel injection pump and refill it - no flush necessary on this.
I'd suggest flushing and refilling the cooling system, too. Be sure to use a diesel engine coolant with SCA's added. Something like FleetGuard or equivalent.
Re-torque the head bolts and set the valve lash, check all bolts on the tractor and you're ready to go enjoy your fully-prepped tractor. If the clutch is working properly you don't have to worry about it; if not, adjust it to the specs in your manual.
