Other factors to consider, how is the equipment used, in what environment, and how has it been stored?
Are you running it hot, and putting away wet? What duty cycle? Humidity will be drawn from outside air, and condense in the warm tank as it cools. Storage is similar in issue. Is it stored indoors or out, protected from weather/humidity?
One option is to pull a uniform sample and have it tested if you have someone locally, or mail in if no one is close. Most testing companies will give results and recommendations based upon their findings.
Here is an interesting article;
http://hydraulicspneumatics.com/200...s/Article/False/6436/TechZone-HydraulicFluids
You've noted that fluid isn't too expensive, thus I would recommend meeting or exceeding the manufacturer's recommendations. You might also consider adding better grade fluid and air filters.
Cheers and good luck.