The big tank is for runnin tree saws! --Got that connection up on my Hi-ranger too! For your use 20 gallons is plenty, and you could use a smaller tank without any problems.
Your clean oil looks great!
Original intended use was more for tree cutting and for extended run time of the hydraulic saws you would need a big tank to help cool the oil.
I figure for our type of use, you dont need that many gallons. Not sure what my Hi-ranger has but looks like about 40 gallons in it, but once the cylinders get filled it dont take much to operate them.
