Rather than Glow Plugs I think the 3520 has the Air Intake Heater device on top of the intake manifold, A single wire connects to the terminal.
You need to verify if the heater is actually working.
Test for power at the connection to the heater when you press the key in. (Multi-meter or Test light)
It would be helpful if you could provide more info.
What's "a certain RPM"?
How old is it?
How many hours? Service history.
These WAMs suffer from overheating and radiator fin clearance is needed constantly.
It's a known issue. Age related.
The crankshaft seal can leak and manifests as you describe.
Not hard to fix.
That gives access to the seal in its carrier housing.
Remove housing and seal complete.
Replace seal. Press fit.
Reassemble in reverse...
The Yanmar engine used in your tractor and many other applications has two fill caps and you can use whichever one is more convenient. They both fill the same sump and there is only one dipstick (Which is visible in your first picture, with the yellow loop handle.)
Sometimes the location of the...