Reply To: need some help! with my gt125rc
Since the bike seems to be set up correctly, I would still suspect the oil. I was not familiar with the oil you used so looked it up at Eurocarparts and it is ACEA A3/B4 car oil (lots of additives for car engine longevity). The oil grade that is recommended by Marcel is JASO MA2 motorcycle oil, he has worked on many Hyosung twins and knows his stuff.
If an oil has additives to improve lubrication/cleanliness etc it can make the motorcycle’s wet clutch slip. Car oil often has these additives as the gearbox is usually seperate from the engine and they have a dry clutch. I have an austin Maxi auto and the gearbox has a wet clutch and shares the same oil as the engine (like a classic mini auto and most motorcycles). As such I have to use a classic low additive oil to ensure the clutch plates do not slip. It is the same principal for motorcycles as the engine and gearbox use the same oil with a wet clutch, you do not want an oil with additives that stick to the plates and allow the clutch to slip. It is always recommended to use motorcycle oil not car oil for this reason. There is lots on the internet about people using car oil and getting slipping clutches.
Halfords have a cheap JASO MA2 motorcycle specific oil, I would change it out and see if it improves. If it does improve it then change the oil again soon, try to think of the first oil change as an engine flush to get car oil additives out of engine. Dont forget the filter as that will hold lots of the car oil also, at the very least remove it and flush it out with the new motorcycle oil.
Here is a short article that explains it. https://itstillruns.com/reasons-motorcycle-clutch-slipping-8739449.html
Good luck and keep us posted.