Brutally hard miles and 3k oil changes? You either went too far on that oil or have a completely different definition of brutally hard than many others. I've ridden over 2k on a RFS KTM without an oil change and I've ridden 100 miles on the same bike where I knew I had to change the oil before riding it again.
Being on a CRF250L or a WR250R does not change the abuse the oil would take. Been there BTW with a WR250R.
Brutal riding IMO includes extreme heat, severe clutch slipping and very low speeds with high drag, like deep mud or sand and nasty hills, if you are even moving at all. It doesn't matter what bike you ride, if you are "brutally" riding it the slipping clutch plates contaminate the oil, the heat bakes it and the stress will just destroy it. How many times in that 6k did your bike boil over? How many times do you need to adjust the overheated clutch?
And the higher performance bike would be less stressed in that enviornment. It was designed for that. These bikes not so much.
This is why it is important to know who is writing a post and how they ride.