ThinkPad fans have a overdesign range of about 2x, they almost never stop
because they're too old. Clogged in dirt, on the other hand... a clogged
fan often spins FASTER and cools less.
Want to see how much your fan has as far as a safety margin goes? use
thinkpad-acpi to measure RPMs in level 7 and also when you switch PWM off
(i.e. set it to 100% duty cycle). Mine gets faster by 1500RPM or so that
reflects the extra torque the EC can apply to get the fan to run at the
speed it wants it to.
But the thermal sink interface to the CPU and GPU are sometimes crap [when
compared with the quality of the rest of the machine]. Open it, remove the
crap, and replace it with a very high quality compound. Dependin on
ThinkPad model, you also need to repair thermal sink design defects to make
it fit properly (read: press really tightly) on all chips.