I have always used 100 mg/l for the total solids limit for hydraulic oils, what is the industry standard for gear oils?

my experience with trending particle counts on gearboxes, etc..., was in usually setting an ISO Cleanliness Target of 16/13 (>5um/>15um) and then establishing a trending of once a month.
in some rare cases 17/14 for non-filtered gearboxes.
Extracting clean, representable oil samples (live-zone oil sampling) was the key though.

If the gear unit had a filtration/cooling unit you can install a sample port directly after the pump by permanently fixing in a tee at the oil pressure gauge with a stauff pressure test port (using a Stauff 1/4 npt gauge adaptor and hose barb, 1/4 nylon air hose tube which is disposable, you can screw on live, purge bit of oil, then fill your oil sample bottle) and make sure its' before the filter, you may also want one after the filter/cooler in order to isolate the gearbox from the pump/filter/cooler when troubleshooting a potential concern.
it ends up being common sense stuff.

If gear unit has only sump, siimply install a pitot-tube affair teed' off the drain plug but guided in as so its off the bottom and in the churning oil. as live zone sampling you can get.

pls note: pressure test ports are also available thru Parker Hannafin as well.