After crashing oblivion few times by using some sort of tweaks programs I decided to take oblivion.ini in notepad. I found a real weird thing called "FpsClamp=0" which I set to 24 and I was surprised how the game works (excluding PC idle animation) even if I set grass distance to max. Also setting "usedrivcache" to 1 was a real improvement. So, looks like is not the game engine the problem for that great pc resources request.
Has anyone some hints about oblivion.ini and tweaking?