250-500cc WR 300 gas mix ???

And in the US 40:1 is 16 oz per 5 gallons

Agreed, I'd mix more oil if I was WFO all the time.

500ml of AMSOIL DOMMINATOR per 20 Liters of 98 OCTANE Fuel - or 40:1

Stiring the pot, adding more oil doesnt mean your making your mix safer for running.
More oil = less gas
Less gas = lean running
You can cause running issues allover the place if you slap to much oul in there.
360 average rider 50:1
My gas gas txt 300 50:1
My whipper snipper 50:1 same mix same putoline strawberry scent oil
No issues yet lol.

More oil makes more power and not because the mixture is slightly leaner, it's because the top end seals up better at high rpm and makes more compression, this has been proven. That said I have run as much as 25:1 but now I run 40:1 because it seems the happy medium, when I ran 25:1 my engine ran better but if I wasn't riding as hard as I possibly could and slowed down to goof off then when the pipe cooled off a bit the excess oil would spooge up the silencer packing. Today's synthetic oils are good enough to run at 50,60,or 70:1 but I want the extra oil for the extreme conditions like deep sand or long wide open stuff.

I run Ammsoil Dominator or Interceptor and sometimes mix the two oils 50-50.

25:1? What oil? Musta been hauling ass not to fowel a plug with that mix.

jetting and oil choice has waaay more to do with fouling than amount of oil. if you do the math, the difference between 32:1 and 50:1 is negligible..

Ammsoil Dominator and it really never tried to foul the plug, the jetting was spot on. It did gain enough power that I could tell the difference but not worth the extra money for using so much oil. I tried the same mix on my yz125 and my 09 WR250... The 125 responded better to more mix oil.

I don't buy that argument at all. 50:1 is 2%, 32:1 is 3%. I have had bikes running lean due to cold weather or due to changing jets; never by changing oil ratios.

Fair enough, but your replacing conbustiable fuel with lubricant so the mix is getting fuel lean.
It doesnt cause seizing but foweld plugs and excess carbon build up specially if your using the wrong oil for speed of riding and if your carbs using same jets as std.
But it is only my opinion.

Changing the mix ratio has very little effect on fuel mixture. Air density altitude is the big variable factor and if you jet a fuzz on the rich side it won't hurt performance but will run better on better air days. You hit the nail on the head with your statement "wrong oil for speed of riding" which is why trials riders can use 100:1 and 125 motocrossers can go 25:1

Beta really has a good idea with their oil injection that can be controlled according to speed, I just have a hard time trusting oil injection after have a mid 70s Kawasaki melt down when the oil injection failed.