Cat 955H oil leak

I picked up a clean Cat 955H in 2017 from a local farmer in Tn, we have run it a less than 20 hours a year usually cleaning roads and moving downed trees.  For her age ( I don't know exactly, I can't find a serial number plate on the engine or rear end) she is very clean.  On a recent trip to our property to work, I started her and within a few revolutions she was idling smooth as usual.  As I did my walk around to check for leaks and anything else unusual, I noticed a small oil leak from the back of the cylinder head between it and the block.  

I'm guessing this is just an old diesel but is this something I need to keep an eye on or just a side effect of old age...check oil level and keep topped off.  

picture of the old girl is attached.  

Hi AM,
as a starting point suggest you re-tension the cylinder head fasteners.
First back off the cylinder head bolts/nuts one half of one flat and re-tension to specification and then go back over the fasteners a second time until all fasteners do not move. Then keep an eye on it, if the leak persists a new head gasket would be required as well as checking that the cylinder liners have the specified protrusion height above the cylinder block surface--if out of spec. something will need to be done to rectify the issue--liner removal and re-shim after checking the liner seats are not cracked or fretted etc. if fretted then the seats need to be re-cut and appropriate thickness shims fitted to come within spec.

From memory the Serial Number may also be on the lower right of the firewall in operators compartment
The engine S/No plate should be on the left side of the engine near the block/head parting line on a flat piece of casting for such--if no plate S/No is usually stamped into the block under the plate.

Cheers,
Eddie B.

Eddie B.,
Thanks! I will give the re-torqueing a shot next time I'm at our property in October. I was going to dig through my 955 manual but you provided the specs as well, very helpful. I really appreciate it sir!

V/R,
Nick