0w20 oil?
 

Since oil viscosity tends to increase with age, would it make sense to top off with 0w20 oil to keep the oil thin especially during start up?

Are you sure the vis increases? As the viscosity modifier breaks down, it should decrease.... For example, a 10W40 will become a 10W30 after 5000 miles - or close to it.
Modern oils are really thin - they even work in Alaska... so no worry, So Cal is just fine. To protect your engine, I would go the other way, from 5W20 to 10W40. Lose the fuel economy - there is none to start with anyway ;) And, if really concerned, use premix in the fuel. Extra lubrication the easy way.... do a search here for the recipe.

Note: This is from memory and a long time ago. I think I am correct.

For conventional oil, viscosity decreases over time from polymer shearing and thermal breakdown.
Note: the w does not stand for weight. The w stands for winter, or something similar.

Using 0w20 for example:
The 0 refers to the flowability characteristics in cold.
The oil will flow like a 0 weight in cold. You want the oil to flow as quickly as possible when you start your car, especially in the cold.
A very thick oil would not make this possible.
The 20 means the oil will be a 20 weight under stress.
The polymers cause the oil to thicken to turn a 0 weight into a 20 weight.
As the polymers get broken, the oil looses its ability to 'thicken' to a 20 weight.
So, as the oil gets older and older.. you have a 0w20.. 0w19.. 0w18.. etc..
So, not only does your oil get dirty over time, the level of protection drops.
A 0w20 conventional oil is a 0 weight oil.

For synthetic, this is not the case. A 0w20 synthetic is a true 20 weight,
not a 0 weight that thickens under heat and pressure.
It just has the flowability characteristics of a 0 weight in cold.

I would not use a 0w20 unless you are in a very cold area.

Hope this helps.. and I am correct :)
-B

I think Polak said a while back that he uses 0w20. You might want to shoot him a PM to see how it works for his 8.

Maybe for performance reasons ?
I'll have to ask him at the next meet.