I don't know if the service requirements for the 180 are different from the 422/425. My manual says 10W40 and the label on my tank says 15W40. Be that as it may, you can certainly go down on the first number if your weather justifies it. But for warm weather and stress ops, me thinks you'll be happier with a 40 or higher viscosity.
I don't know if it's different on your unit, but the 422/425 PT specifies only a FILTER change at every 50 hr. The manual states </font><font color="blue" class="small">( Change the hydraulic oil only when it is discolored or contaminated with water or other foreign material. )</font>
The hydraulic reservoir tank oil does not need to be changed in any prescribed manner based on operational usage, only the filters at every 50 hr. Changing non-contaminated hydraulic oil is optional per owner. imho, I would select a good synthetic oil with a viscosity range that covers year-round usage, change it once and be done with it unless a contamination problem develops. In terms of timing, some on the forum recommend doing this at 100 hr, but timing is also optional on this.