I’ve only seen
Hurth mechanical transmissions on the back of
Perkins 4-108 ( not hydraulic Hurth) and they have an oil cooler just bolted onto the outside of the
gear housing, just an alloy tank with an in/out hose. A failure into the oil side would have to be through the
gear case itself , not just the oil cooler.
The often used Borg Warner transmission however does have an externally mounted tube type cooler that fails ( usually because the anode has dwindled) could your
Perkins have this transmission?. If so, the capacity varies according to the configuration.....direct drive: reduction drive and vee drive. Here’s the recommended technique for checking oil level.