My L245dt leaks oil around gear leaver when it is under load is it just an o ring and what are the inlet & exhaust valve clearences

Probably defective valve stem oil seals allowing oil to leak along the valve guides and enter the combustion chamber/exhaust system.

Valve stem oil seals tend to be more affected by time than mileage and some of the materials used have been shown to be adversely affected by some of the oil additives used by oil companies.

One further thing to note - inside the modern engine a large volume of oil is delivered to the top of the engine (valve gear) every minute and in order for the oil level around the valve gear to remain constant the oil must drain back into the oil pan at least as quickly as is delivered. A too high level can overwhelm even good valve stem seals.
Sometimes the oil return ports can become restricted...

An engine does not make oil, but it can spill the oil that should be inside. You could have an oil leak, which means that oil will be leaking from either the front or the rear oil seal. The front oil seal is just behind the crank pulley, the one that drives the fan / alternator belt. If oil is leaking from there, suspect the front seal.
If you have oil leaking from the bell housing, that the the bell shaped metal thingy between the engine and the gearbox. you can suspect the rear oil seal.
If you inspect the engine and see oil leaking from the oil pan under the engine, you have a leaking pan gasket. This can usually be solved by tightening the pan bolts, a whole lot of bolts underneath the engine.
Finally, and this is probably the worst, if you see oil around the breather pipe that goes from the valve cover on top of the engine to the gas intake system, you have leaking compression, that means either the valve stem seals are leaking or the compression rings are finished, and you will have to have the engine overhauled, or at the very least have the cylinder head overhauled.
If the rings are suspect, you will also have excessive smoke out of the exhaust when you drive, especially when you accelerate. Check the exhaust for signs of wet oil.

year, make, model, engine size?

How does it run?

Do you see a lot of black smoke out the tailpipe when you accelerate?

do you see anything leaking?

I’m happy to assist further over the phone at https://www.6ya.com/expert/jeremy_d728a59f986299fa

As a rule of thumb most valve clearences are .010" inlet and.012" exhaust ( hot) but I suspect yous will have hydraulic tappets which take up the clearence as soon as the oil pressure comes up

12th inlet and 14 or 15th exhaust will do nicely

Have your valve seals checked, Mits is back on valve seals leaking and burning oil.

valve clearence-INLET 0,15+-0,8
valve clearence-EXHAUST 0,30+-0,8
hope that helps you mate

rule of thumb for most engines 10/12 th inlet 15/17 th exhaust ,like 25th for the plugs ,4th per inch of the diameter of the piston for piston ring gap ,water cooled 20th for air cooled engines ,1342 firing order or 153624 ,cannot remember the 8 cylinder firing order

interanl oil leak. possible bad valve guide seals or a broken piston ring

First you need to determine if the van is "burning" oil or "leaking" oil. If the exhaust is bluish in color, this is a good indicator that you are burning oil and you probably need a valve job. A common issue with vehicles this age is an oil leak around the valve cover gasket. Oil leaks and burns off on the manifold. If when you open the hood and smell a great deal of oil, this is more likely the cause of using so much oil. The best way is to inspect the engine compartment first. If you can't spot any excess oil there, get under the van and look for a leak near the oil pan, oil plug, or more forward toward the engine compartment. Hope you find this answer helpful. Best of luck.
Greg