[PerazziMx14]

You used oil which was much thinner than required by your bike, before buying the 15W50. Why? Maybe this caused wear and now your engine consumes oil. 5W40 is definitely not the correct oil for this bike. I use regular 15W50 motor oil, as the dry clutch design doesn’t require motorcycle specific oil. Mobil1 15W50 full synthetic is what I buy but it doesn’t matter much so long as the viscosity is correct. Which in your case it wasn’t correct for I don’t know how long. Maybe the damage is already done and you need to have a mechanic look it over at this point.

While 40 weight oil is thinner than 50 weight it's not "much" thinner. The 1st number is much more irrelevant as the is the flow characteristic of the oil at freezing. Looking at Albanian weather it looks like average high temps in the summer is 91 degrees fahrenheit so 10W-40 really is not too far of a stretch, especially of the OP is not routinely riding in stop-n-go traffic in peak heat.

However with that the OP is talking about excessive oil consumption using 800ml/27ounces per 1000km/600miles is excessive. At that rate his bikes exhaust should look like a 2-stroke running 32:1.

If it truly is consuming this much oil there is a much bigger issue that changing from 10W-40 to 15W-50 did not create.

 

[PerazziMx14]

He used 5W40, not 10W40. It’s just not the correct oil and I wouldn’t use it unless I’d plan an ice road ride in February.

Again the 1st number is not that relevant as it is the flow characstic of the oil at freezing. The 40 weight while not ideal is not the end of the world especially. Given the information we have been given I do not think the oil was the culprit. Of course we do not know if the bike had overheated or been beat to death only that once the oil was changed it started consuming 27 ounces of oil every 600 miles.

I do like the OP's phonetic spelling of the "F" word

 

[PerazziMx14]

Put the hot bike on the side stand overnight and start it up. Watch the exhaust the moment it fires up. If you have oil smoke (easy to tell from water vapor) for a little while, then valve stem seals come into question. Before you go any further, check the crank case ventilation for function. AFAIK there is a valve in it, that can get stuck and the engine pulls oil through the intake into the combustion chamber.
If that all checks out good, piston/cylinder wear need to be checked. Instead of a compression test, have a leak down test done because you can precisely tell where the pressure loss occurs. Noise in the air cleaner, intake valve seats leaking. Noise in the exhaust, exhaust valve seats leaking. Noise in the crank case ventilation, excessive wear piston, rings cylinder bore.

You need to work systematically with the right mechanic or you lighten your wallet for no reason. Good luck

If the OP is burning 27 ounces of oil every 600 miles he should be able to look at the exhaust and see a tremendous amount of black oil residue running down the outside of the muffler and notice an absence of mosquitoes wherever he rides.

 

[PerazziMx14]

True, but that doesn't tell him the cause of the problem

I'm suspect of the rate of consumption. 27 ounces is 20% of the oil volume.

 

[PerazziMx14]

The cat takes several minutes to heat up completely. Again if the bike is using 26oz per 600 miles the exhaust would look like Uncle Bucks 1977 Mercury Marquis Brougham belching out a white plume of smoke.