Thanks guys.

I notice the oil can look fairly clean on tne dipstick but after draining its really black.
I understand that the cleaning agents in the oil are extracting the carbon but I'm fairly amazed it gets so dark.
My fuel economy seems excellent so I assume this carbon is "by design" and is not a problem with the piston rings.
I understand that unburnt gases are re-routed back into engine as an emission control, so I expect a lot of it comes from that.

I've had a look at what is involved in doing my own oil changes. Seems I need a floor jack, couple of jack stands, possibly ramps, couple of wheel chocks, oil pan, gloves, filter removal tool, rags, sockets and possibly a breaker bar on hand.
I'm in an apartment these days and not much space for these things, either for storage of tools or working space around the car. Would probably take a year or two to recover the costs too. I dont think its worth it for me to tell the truth.

My current regime is to fully change the oil and filter every 6 months with a second oil change every 3.
I'm not doing enough driving to base it on k's. For example, I would never get near say 3000 k's by 3 months.
My mechanic does the drain, flush and filter at the 6 month mark while I simply siphon out the oil at the 3 month mark and refill it.
The siphon, while slow, is great! It pulls nearly 5 liters out which must be near full capacity. I assume the filter has a bit left in it with this method, but I'm also using full synthetic long lasting oil.

I considered skipping the 3 month siphon since I'm using brand synthetic, but its so easy and the oil does get pretty dark by that point, so why not? If it looks after the engine better then its a small cost in time and money.