Key reasons for oil consumption on the engine side is oil getting into the combustion chamber. The primary two routes for this are past the piston ring and past the valve guides.

Piston rings:
The piston rings will never seal perfectly in the cylinder bore. Example; gaps have to be left in the rings to allow for expansion as the engine temperature rises. This means that oil does get past them to be burnt off in the combustion chamber. There are a massive number of contibutary factor to how much gets past. Cylinder bore geometry, ring tension, surface finish, honing angle, ring gaps compression ratio, chamfer sizes on the piston ring grooves etc.

Valve guides:
You need some lubricant for the valves to move backwards and forwards and the intake system is normally at below atmospheric pressure this draws oil past the the valve stem seals and into the intake system meaning it can be burnt in the combustion chamber.

On the oil degredation side I'll bow to Si2k on that side

 

I think that's to do with the hydrocarbons breaking up over a period of time due to heat, Mobil 1 is a case in point.

 

Oiljets

 

Its the Vtec Pixies

 

Oil vapour being fed back into the intake is good for enviroment but not good for engine, too much and it causes knock making the ECU retard the ignition, a good catch can is worth investing in if oil consumption is excessive

 

Oil degredation is the key factor in my S2000 using oil.

As long as I change oil every 4-5k miles, I never need to top up inbetween.

 

I have a 99 which in the past has used an average amount for its age, but since switching to Castrol it uses a lot less.

 

S2k_nut has also reported the same.

Oil's I have used

Mobil Super S (Semi Synth)
Silkolene Pro-S 5-40

Castrol Edge 5/30, 0/40, 5/40 - these have all been the same in terms of useage.

 

I also run a 99 . Changed the oil filter and oil ,{ Castrol TD 5/40} two months ago and have had to top up more times than I did in the last four years. { Castrol 10/ 40 } .
Checked for oil leaks,pcv valve is ok . So maybe it is the vtec pixies !!!!!

 

If you change the oil every day then you definately wont need to