I use the cheapest synthetic oil I can find. It is usually walmart supertech. But sometimes the auto part stores have specials. I then buy a good filter which is usually purolator pure one filter. But I have used the cheap walmart one and nissan filters along the way. I drive alot of highway so changing oil sooner would be a waste of time and money.
As for not changing the manual tranny fluid, the transmission shifts great and I always expected to need a clutch or axle by now. I wait until that happens to change the tranny fluid since draining it is usually required for those jobs. Just my luck the clutch is still perfect and axles are holding up.
The coolant most likely should have been changed but again no issues with the cooling system as of yet. Does coolant go bad? Some manufactures do not recommend changing it until well after 100k. So I was waiting until I needed to do it like if a hose split or the tstat needed changing. Again none of this has happened to me yet.
Spark plug change? No missing, engine idles great, accelerates great . . . well it accelerates like new. I wouldn't call it great but it's a gas sipping 1.6. So no need to change the plugs.
The serpentine belt def needs to be changed. It has cracks in it. If I do anything this will probably be the one.
I always say I will do all these things but life has been super busy. I was selling a house, had a baby, bought a new house and am now moving in. When the dust settles I may tackle these things but honestly I am just waiting for the car to die so I can buy a truck (see signature). This vehicle was supposed to be a budget A to B and it exceeded expectations. The car was FREE!! The money I saved in gas from my old car paid for this vehicle. If it died tomorrow I would not be upset at all. Even if it's death was premature from me neglecting it.