Losing Viscosity

What oil are you running? What are the operating temps? Any othr info you can supply could be helpful.

Omala 320 @ 37deg C.

Monograde industrial gear oils do not shear in service. It's likely that someone has been topping up with a lower ISO viscosity grade oil.
Omala 320 should have a viscosity close to 320 at 40C so the target of 240 does not correlate with the viscosity grade of the oil - it's closer to 220.

High shear can cause permanent viscosity loss. Using a heavier viscosity than recommended at room temps can cause excessive shear. If your oil supplier is nothing more than a distributor, typically they have very little opereational knowledge. Where are you located?

Dawson;

As I can pick from your original post, you attempted to raise the oil’s original viscosity with heavier oil. If that’s the case, then your calculated amount of heavier oil to be added was wrong. I’ve never heard of the equipment and its operational conditions that would cause any oil to lose its viscosity to shear in such a short time, unless it is a case of a weak and unstable viscosity improver’s polymer (which could settle out of oil if mixed with seriously incompatible oil).

However, it would be nice to know what system you’re talking about and what is the rational that you took “advice” from an oil marketer to spike your oil and change its viscosity. Is it that you wanted to meet viscosity the original equipment manufacturer recommended, or you guys are experimenting something?