Welcome, I have a 2018 Gen 3 - not quite the same as yours, but having changed an alternator/ stator on a Triumph 955i before there shouldn't be any oil behind the alternator housing. I've just checked the K46 2017 Haynes Manual for my bike, the photos show it to be dry of oil behind the cover and I am assuming that your model is not radically different. Wait for someone else to come along here with the same model as yours and correct me if necessary.
The Haynes Manual recommends cleaning the mating surfaces thoroughly of the old sealant (don't scratch those surfaces) and then using a Dow Corning 07580397777 fresh sealant or equivalent and use NEW bolts. There are long and short bolts, so make sure you remember what length goes in what hole.
"Tighten the NEW bolts in a clockwise direction starting with the bolt that is next to the clutch release arm to 3Nm, then tighten them further in the same order through 90 degrees"
I suggest you consider putting a GB Racing or other alternator cover on top as you fit everything as they provide new longer bolts with their covers as part of the kit or buy new bolts from your dealer.
Best to remain waiting to see if anyone else in the know on here would correct me here for your model?
