well to keep it from happening is tuff if you keep it outside , if you can keep it under cover as a roofed building not under a tarp will be best. these systems breathe so the climate that you are in determines how much possible consendsation you might possibley get. I would get the cheapest oil and filter that is suggested for use in your machine empty what you can ,refill and be prepared after a few hours of running, to do it again. and look and see if you have made progress on what you see, and possibly do it again. I do not know how tuff those machines are to purge the air out of so that is something you will have to weigh out if you remove all the oil.
I’ve discovered three possible issues with my L555 that I’m hoping will reduce the moisture problem. The side panels need to be very well sealed. I found water could enter mine through the chain adjuster (one appears to be incorrect for my model, so this is unlikely to be your problem). Check for a good seal here too. The hydraulic tanks breathe through vents located on the inside walls of the cab. If these are blocked, condensation is more likely to remain in the tanks. The L553 shares a lot of features with the L555.
If you have hydraulic attachments you can also introduce/re-introduce the milky fluid when you use them. You’re fighting condensation no matter what. My 1992 785 takes 20 gallons. I just had the chain cases cleaned after checking for wear. The gears, chains and cases looked brand new with the covers removed.
the return passages were vacuumed out. That left the lines and motors as the only fluid left. That was enough to make it look like I didn’t change any fluid At all after I ran the machine twice. Coming up on 5000 hrs. I got it with 4000 hrs and have owned it for nine years. It’s just home owner used and it’s a beast. Fluid never really looked anything but milky(ish).