welcome to the mad house and congratulations on buying your bike at the right price. I have one piece of advice for you, change the oil, as you have suggested, to do this properly it meanns dropping the exhaust and removing the strainer from the underside of the crankcase (the door for which has 3 unequally spaced bolts), clean it in petrol until it sings. put everything back and (of course) fill with any good quality 20/50 preferably suitable for diesels, the do NO MORE than 150 miles and repeat the process, if you see any fluff on the strainer gauze, you have done youreself a favour and discovered that your choice of new oil is not compatible with the unknown brand that is in it, and prevented cam seizures, when you fill with the second amount of fresh oil you will have nothing to worry about (oil wise) if you dont see any fluff after the 150 mile change, then you have proved that all is well with your new oil, as it will be in every case that you do a second change, (does that equate to waste? I think not) then stick with that brand, but if you decide on a change, repeat the process.
CLEM
250,000 kilometre 1200TS and no external oil cooler! owned by me for 43 years