I am going to clean my computer. First I want to oil my fans (CPU, GPU, and case fan). I don't have any machine oil. Can I use oil from kitchen (sunflower or rapeseed) to oil them?
If you're looking for noise reduction, lowering the fan speed is the best method. Fans are cheap and putting moist things inside them (like oil, though it technically isn't a liquid) may result in temporary improvements, but these are offset by problems from attracted dust and the like.
The nicer fans don't even had a physical connection between the blades and motor, and the spindle and fan housing, as they use magnets to hold the two units separate. No touching means no lubing.
Vibrations in the case are due to imbalance, not bad contact. And imbalance is fixed using a file (if you're patient and OCD) or a credit card (if you're not). Higher quality fans are better balanced, and and magnetic bearing fan with the maximum possible diameter will allow for the quietest long-term operation.
NO NO NO NO
Cooking oil will "gum up" after a while and the fans will literally stop spinning.
Machine oil is better, but even then, it can catch dust and ultimately increase friction.
Your best solution is simply to replace the fan, if it is noisy.
Rapeseed Oil is the gummy junk I have to wipe off my glow fuel powered RC airplane after flight so my hands don't stick to it.
I can attest to its tenacity, near unremoveability and stickiness. In the presence of alcohol as a solvent, it is an oil lubricant. After it has been liberated from its solvent, it becomes gummy, after some exposure to air, starts to polymerize and turns into an unremovable varnish.
Good luck with using it. As with most vegetable oils, you will find out why we quit using them and things like sheep fat to lube our cars a century ago.
I'd recommend a mix of 3in1 oil and Triflow teflon if you're really serious, but most fans that have started to make noise have sleeve bearing damage or were cheap ball bearings that have started to disintegrate and don't last very long after you oil them.
Replace the complete fan with good quality ball bearing fans by reputable manufacturers that know how to grease them for long life.
We use oil to reduce friction.
Up to some extent we can use sunflower oil, but if you can arrange some drops of bike engine oil or any vehicle engine oil then it works better.
If you have a bike, then after servicing, its engine oil is replaced. If you have that, it would be good.