bon amide is a bit harsh but it was a old bush trick to "fix" a sad engine.

the only fix is a rebuild but since it runs well the fuel system needs very little attention. but everything else needs some attention. you may be lucky and the crank doesnt need grinding and only needs new bearing shells. it certainly will need valve guides and probably new valves and you grind the seats too try florin tractor for the parts as they may be able to supply aftermarket parts. note some cat engines are actually perkins engines and that is another source of engine parts. but the only way to tell what machining is required is to pull it down so these things can be measured.

or as others have said just keep topping up the oil. although that gets old fast.

Had a 3304 DI doing exactly the same and burning oil faster than imaginable but started easily and ran nicely otherwise.
3304/6  parts are plentiful and well priced. I'd recommend new piston and liner kits and a head rebuild at least. 
My experience is that provided  you have the lifting equipment its less frustrating to pull the engine and work on the floor , even if the crank checks out ok.

I disassembled the oil cooler above the filter. it turned out completely clean. The system had a clean blue coolant. There is an oil cooler on the side of the water cooler, all clean. I must say that the system does not heat up and the indicators always remain on the green. My doubt remains. Can enough oil pass through the valve guides to end up in the combustion chamber? Or more simply it is a question of rings. because remaking valid guides is much cheaper and even simpler. This is the question, removing an engine is never rewarding. Removing a cylinder head is easy.

you answered most of your questions....i agree you are correct and you want confirmation..

no to Bon Ami trick....did that many moons ago..(gasoline engine) are you kidding me, that doesn't restore clearances..

i know its not what you want to hear. but a rebuild is the only fix. yes you could start with a head rebuild but if there is alot of sideways play on the pistons the the rods and liners have to come out and really its not much more work to do so its just cost old cat engines are fairly simple and remember your not touching the camshaft or injection system as it runs too well for that to be a issue the lift pump is easy to check and fix without touching the injection pump itself.

the amount of oil consumption says its not just valve guides, the rings and liners need changing too, it may have faulty oil rings one could be broken the only way to tell remove and inspect.
there is no way to tell with out dissasembly.
one final piece of advice get the service manuals for the machine and read them in particular the one for the engine before you even start the job a manual is the first place to start before you do anything.  heres a tip when it comes to doing heads and bearing caps number everything so you put matched parts back together it makes reassembly easier.
when doing valves i piece of scrap cardboard is handy punch 2 rows of holes number them inlet 1,2,3 etc exhst 1,2,3 etc then you can keep the valves and springs all in order unless you are replacing them with new parts. and have plenty of containers to put bolts etc in so you can keep them separated.

these are things i have learned from experience and from others who learnt the hard way. and from pulling apart my sisters toys as a kid i couldnt always put them back together in working condition but i learned how to fix em.

I'm no Mechanic, but have pulled a few Cat engines apart, including two D4 engines that got new liners, piston rings and reconditioned heads, once you pull the head off, a job that should only take a couple of hours depending on the Cat, you are potentially 50% of the way through a in frame engine recondition, because if the cylinder liners are OK, it's just a matter of pulling the pistons, if they look good, then it's probably just a set of rings needed.

On my old D4 engines Cat say you can replace the conrod bigend bearings without the need of removing the sump, in theory they can be done through the 2 engine inspection plates, I wouldn't want to do them that way, but Cat say you can, so in theory you could pull the pistons too without removing the sump, but the extra hour work it takes in removing a sump on a Cat without a sump guard, I wouldn't muck around trying to do it through the side covers, pull the sump off too.

Listen to the experts on here like "edb" and "Old Magnet", my advice is pull the head, recondition it, and check your bores for wear, remove the sump, pull your pistons and at a minimum aim to fit new rings, if parts are badly worn like the pistons and liners, go the extra step and do it right and replace them, as "Glum" says, those parts are not hard to find and probably not very expensive.

I'd be surprised if a engine could suck 10 litres of oil down the valve guides in 4 hours run time, but as I said before, I'm not a Mechanic

Hi uisum,
you say the oil usage started suddenly.When looking thru the 3304/6 Systems Operation Testing And Adjusting section of the Service Manual I see Problem # 16 of the troubleshooting (Problem finding Charts) is Titled Oil At The Exhaust.
See scan below.
It lists too much oil in the valve compartment as being one possible cause which could be due to a core plug, one situated in each end of the valve rocker arm shaft, being blown out/missing thereby flooding the valve compartment.This coupled with possible worn valve guides could be an easy fix.

Cheers,
Eddie B.

good idea edb and that explains a sudden increase. and very easy to fix with a new welsh plug sealed with litharge(yellow oxide powder) and glycerine you make a paste smear the plug and recess and install it let it dry the final cure is from heat. or use "stag". the other method is to tap it for a threaded bung you just pack the drill and tap with grease to hold the filings done it a few times works well makes tapping blind holes much easier.