I changed the oil in the Leimbach powered excavators bi-monthly b/c I used them nearly every day, in the basement they kind sit inside a mini dust cloud as they work and being the cylinders are made of aluminum, the aluminum material wears a bit under higher pressures. I think the 4200XL hoe's cyls are probably aluminum too.
I would lift the boom to the top. Shut off pump. Operate the valves through the radio & by hand articulate the bucket & stick cyls 1st to squeeze the oil, then lower the boom last.
I use a veterinary plastic medical syringe with a 12 or 14 guage needle to suck the oil out the tank (they cost about 10 cents each). Be sure to suck as much oil out as you can before you start emptying cyls or you'll have a mess overflowing the tank.
Then use a clean syringe to refill. The tank on the model is not big enough to refill all cyls at once, you'll have to keep adding. Using a lower voltage battery while refilling oil system, like 6 or 7.2v, the pump will turn slower, creating less foam.
The 4200XL does
not come with a filter. My Vario hoe I think is using fuel filters, the Vario company's mainline models are helicopters afterall. The Vario uses two, one for each return line, and the 4200XL has two return lines as well.
How often you change the oil depends on how much you use it. You can check quality & color periodically with sucking some out with a syringe & decide for yourself if it should be changed or not.
I use a 10W Hydraulic oil. I use my models in a room temperature 15 - 18C or so.
If you're using your model in direct sunlight at temps of 25+, then you would better with a 30W Hydraulic oil.
I've heard ppl say that they use ATF or engine oils, but you're taking a risk of a lower purity oil of buggering shyt up with leaks & loss of pressure. Hyd oil costs a little more & might be a special trip to a petroleum dealer to get it, but you're not using that much anyway & a gallon jug will last you a several yrs anyway.
The 4200XL doesn't have any bushings anywhere (not that I've noticed in the latest models, the early one didn't), it'll be steel pins in the aluminum plates, not much surface area to wear against... you could install your own selfmade bushings now or do it when the joints get wobbly -> your choice.
My Vario uses a steel pin with a hard plastic bushing & very little play in any of the joints 3yrs later with over 10cu yd of virgin digging moist brown clay.