This question sounds stupid,but is the oil level correct?
Can you check if the dipstick is the correct for that model or mast?
Had similar issue three weeks ago in a JCB telehander gearbox.Some "clever"guy swaped the dipstick for a smaler one from another model and i only noticed when was filling it.
Also check the return lines and filter inside the tank.
Hope this helps a bit.
Best regards.
R.Feitor
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