Topic approved.

Hi Fernando,
The coolant issues are very common in KTM. But thankfully, they are easy to fix as well, unless its something catastrophic.
Seals are common culprits which you had already replaced. They lasts long in most cases, so should not be the issue. If the coolant mixes, the colour and odour of engine oil changes. The oil filter paper element is in bad shape due to that.
As you have no coolant mixing with engine oil, first thing you can do is check if its burping the coolant while you ride your motorbike. Yes, seems very unusual suggestion, but i have observed the same in my case. It may be a different issue in your case, but let me share my experience which might help you diagnose the issue.
I hade this ride planned in Rajasthan, where I took my KTM duke 390 2014 in hot Jodhpur to Ajmer highway. I observed that while my engine temperature was in check, the coolant had burped and gathered in reservoir tank above during the ride. This is common, but in my case, that kept getting collected and maxed out the tank. Due to this, the tank started loosing the coolant from reservoir overflow hose. The amount of coolant I lost is the reason coolant seen less in the reservoir when it settled down after the ride(when engine cooled down). In my case i had to manually empty the reservoir tank, and then fill the radiator with coolant(it had less coolant and was not fully filled). After that, i filled the reservoir as well. Approximately 10 20 ML coolant was lost due to burping and I topped it up with same. I'm sure in most cases, it goes down automatically when engine cools down, filling the radiator again.
see to it that your coolant reservoir is not loosing the coolant while you ride, or notice the level during the ride itself.

rider_soul Ohh, thank you very much for sharing this. I'd also like to let you know that I recently got my Duke 390's head gasket, block gasket and water pump seal replaced again. But the mechanic told me everything was absolutely normal and there were no problems with the gaskets nor the seals. And yet after doing this my coolant is dropping again slowly mostly if I ride for longer durations. So I'm assuming its the same thing that's happening to your 390, I shall give you an update once I find out if it's the same issue as your bike, thanks a lot

Fernando0711 Thanks for the additional info. Keep an eye on normal evaporation of coolant. I use Motul one which is good but not as good as Engineice. I'm planning to upgrade. Talking about my issue, it feels that you might not have the issue that i faced and this I can confirm after some recent investigation by me.
Lets talk why coolant didn't go back to radiator. Coolant level can only not go back to radiator from reservoir if there is pressure leak inside. Ideally when the engine cools down, the coolant which had expanded due to heat cools down as well. This should create enough pressure to suck coolant down the radiator cap. There is small cavity in radiator cap which opens at low pressure inside and thus allow the coolant inside. When I investigated, I found that gasket was okay but the left side radiator hose was rubbing against engine part, and got a small hole init. Though I didn't see the leak there but the pressure was leaking from there I believed. This surprisingly sad as hose was running very close to hot engine as well as touching it. I'm not sure if this connection is fixed in 2017 and later models or was it only me who encountered this.
I Fixed the patch temporarily for now and rode the bike till it got heated. Later the level of coolant dropped to original level. I'm in a remote part of Rajasthan where there is no SVC till 80km. I will ge the hose pipe changed soon.