The computer muffin fans supplied with most equipment are the cheapest sleeve bearing fans that they could buy in bulk at the time of manufacture.
Yes the sleeve bushings can and should be lubricated, but this is temporary. How temporary? Impossible to say.
Compacted dust on the fan blades is certainly an issue which reduces airflow. If the fan speed is variable with heat, then they will have to run faster and louder for the same level of cooling. My experience is that large dust accumulation slows the fans and makes them quieter. Mostly the quieter is a lower pitch but the measurable DB is likely the same.
Some companies Like Noctua, Silverstone and a few others have taken computer fans to a practically ridiculous level.
I'd Say Noctua is top dog in efficacy for the least amount of noise.
They have a 7 year warranty, if that is the type of thing to float your boat.
I have 3 Noctua fans employed at this moment. 120MM, 80mm and 60MM.
Most will balk at their price. My 120MM Noctua moces 55CFM for very little noise and only 0.05 amp consumption. that is less than half the consumption of other fans moving the same amount of Air.
WD-40 is best used for its designed intention, water displacement. It is a horrible long term lubricant, and in my experience has no ability to prevent rust, and it will eat rubber seals despite the claims on the can.