Can AVGAS 100 LL be used without any problem in Rotax 912i engine which suggests a fuel with RON 95 or higher? What would be the advantages of using AVGAS over unleaded RON 95 gasoline?
Yes it will run fine, but...
Lots of drag racers use Avgas in their cars to get the octane rating to run 12 - 14:1 compression ratios in high performance engines (At the airport I used to hang out at in the 70s, a Corvette owner that dragged his car would stop by every weekend to fill his car with the old green 100-130 octane fuel, because it was the only fuel he could use without blowing his engine up, his compression ratio was so high, and the car having old fashioned points ignition - no knock sensors or any of that fancy stuff).
But you can expect more frequent plug cleaning due to lead fouling (although this might be not too much of a problem with the 912's compression ratio).
The biggest thing to beware if is the oil. If you are a using full synthetic motor oil, this can be a problem because the polyalfaolefin base oils of synthetics are unable to hold lead combustion byproducts in suspension in the oil, and they will settle out in the engine instead in a sludge that coats the inside of the engine called "lead paint" (one tank full would be no big deal, but long term use could ruin the engine).
There was a big class action against Exxon Mobil years back by engine owners suffering from lead paint problems inside their engines (including a few failures from plugged oil galleries IIRC) after running them on Mobil AV1 full synthetic oil, which had to be withdrawn from the market (I used it briefly in my Cardinal).
The Aeroshell Multigrade oil line is only 50% synthetic (needs to have mineral oil in it because of the lead issues) and works well with avgas, and the other aviation oils are generally mineral based or no more than half synthetic.
So, bottom line is, if the engine has full synthetic oil (probably an automotive oil), it's ok in a pinch, but avoid running too much Avgas through it, and if it's mineral or semi-synthetic, should be no problems other than the potential spark plug issues.
Beyond that's there aren't really any advantages in the short term, unless you need the lower vapor pressure characteristics (higher boiling point) of Avgas (more resistant to vapor lock when it's really hot under the cowl) for very hot weather operation or operation at high altitudes and high temperatures, where mogas boils more readily. The other one is long term storage. With mogas it's a good idea to use fuel stabilizer if the plane will be parked for more than 6 months with mogas. Avgas being paraffin based, is very stable and can be stored for long periods without breaking down.
I use mogas in my Lycoming O-290 engine and it is much happier on that than on Avgas, which like to foul the lower plugs after only about 25 hours.
This question comes up regularly in automotive circles.
There is no advantage to using fuel with higher octane number than your engine requires, though in automobile engines there may be a minimum and an optimum (the car computer can adjust spark and, in recent years, valve timing to optimize efficiency, and will use higher compression and more advanced spark, both advantageous for power production and economy, on higher octane fuel). This is not the case with any aircraft engine I'm aware of.
So, your Rotax 912i will gain nothing by using 100 octane avgas compared to 95 octane auto gas. What you may find (assuming LL stands for Low Lead) is that the 100 LL will result in somewhat shorter spark plug life due to something automotive workers haven't had to think about in forty years: lead fouling. Whether this is a problem or not depends on several factors, chief among them the type of plugs used in the Rotax 912i (polar gap or "conventional") and how long they're left in the engine before changing them.
Beyond the possibility of lead fouling, however, 100 LL avgas will not harm your engine (running 80 octane avgas might, on the other hand). I'd suggest you run whichever is less expensive, keeps your engine cooler, or gives best power and/or fuel economy, as suits your needs.
I'd say the fuel feature ON indicates is its resistance to detonation, self-ignition, but energy content of AvGas being same as in any vulgar low ON gasoline, as the 85, or even 76 RON, the only available in some places. Detonation can destroy an engine very fast, but if the producer indicates your engine can work on ordinary fuel, using AvGas would add nothing but expenses to your flight. The issue is very important, and in doubt about ON of the fuel they offer you, AvGas could be sometimes a good choice. Blessings +