So the owners manual is not clear at all, but I'm assuming that you check the oil level with the engine off and the bike on the side stand, correct?

I say this as the 3.5L capacity that the owners manual states completely fills up the oil view window when the bike is upright or on the center stand.

Did you change the filter?

The bike should be upright and level and the oil aligned with the mark beside the oil window. I've always done this with the bike on the wheels and just held it there...

I ask about the filter because if you swap filters and put oil in but haven't started the engine yet then oil hasn't been pumped through the filter yet which will hide some of the oil volume inside it...

certainly do NOT fill up to the mark with the bike on the side stand - that will be overfull.

As Bluey says, hold the bike level and upright and eyeball the sightglass. When changing the oil & filter; I fill the filter with oil before fitting so that when I start it oil pressure gets up quickly. Run the engine for a minute or so to get the oil circulated around it then switch off, let it stand for a minute then check the oil level again, topping up as necessary to get it towards the upper mark. It should not be higher than that.

I changed the filter as well, and let everything drain for a good 20 min so should have been bone dry.

When you do your oil changes, are you putting in 3.5L of oil? I put in exactly that amount, which is the listed capacity in the owners manual, and it completely fills the view window when the bike is upright.

Here's a picture of the sight glass when on the side stand. There's no min level and the max is crazy low. Do your sight glasses look like this too?

So I was talking to a friend of mine in the motorcycle industry. Apparently that max line is a sticker that was applied to bikes during this time period. It seems that a common complaint at the time was that the Ducs took a long time to warm up. The Italian solution was to drop the oil level...

I never put in the whole 3.5 litres. After draining the oil and changing the filter (filling the filter before attaching) I fill the oil until it is half way up the sight glass. After running the engine and letting it settle for a couple of minutes I top up until it is again half way up the sight glass (where it should be). This usually uses 3 Litres or very slightly more. On a normal change there is always some residual oil left in the engine. The only time it will take a full 3.5 litres is with a completely dry, say newly rebuilt, engine.

I agree. Bike upright and half way up the glass.