The main reason oil goes bad is because it's viscosity breaks
down. In other words it gets thinner. Heat cycles break down the
viscosity. The more the oil goes from room temperature to operating
temperature to room temperature again the faster it looses
viscosity. The limiting factor would be the oil filters ability to
keep the oil free of dirt/deposites. If a car is taken on alot of
very short trips the limiting factor will be the viscosity because
of the heat cycles. Your engine manual will give you recomended
changing intervals. ( The handbook is just a guide)
Synthetic oil is not always desirable because of it's inter action
with some material. Do some reaserch on your particular application
and always consult the owners manual.
1. 5W-30 is around 60 cST at 40°C and around 12 cST at 100°C.
2. 10W-30 is around 66 cST at 40°C and around 12 cST at
100°C.
The generic interval is 5000 km + - but the true interval is found
in the owners' manual. Second, your particular type of driving
affects when it is time for an oil change. If most of your driving
consists of short trips or commutes (under 30 km) and a lot of rush
hour, stop-and-go driving then your oil changes are under the 5000
km "severe" service schedule. Oil needs to be hot (about 1500
degrees) for maximum flow. Short trips and frequent stops do not
allow for ideal oil circulation to moving parts inside the engine.
If the car is used primarily for short trips ( city driving), or
routinely operated in dusty conditions or slow-moving traffic, the
oil should be changed more frequently. In general, changing the oil
at more frequent intervals will help better protect the engine, and
promote longer engine life. Oil should be changed every (5000 km).
However, if your commute is mostly highway driving greater than 30
km + -, one way with few stops and outside of rush hour traffic,
oil changes can be extended a bit (about 5000 7000 km). Finally,
if your engine is serviced with synthetic oil your intervals can be
extended beyond 5000 km + -
To each it's own ! logically speaking changing of engine oil at
every 5k interval is good for you car, especially in this local
climate. Is like changing new or fresh blood just at the Optimal
level, is nothing under Optimization / usage. What do you gain of
stretching the oil to 7,000 km-10,000 km. Compare to you purchasing
your ride at $50k-$80k for your March vs servicing cost of a few
hundreds to maintain your car for a period of 3-5-7-10 years. Your
call...! 
Dun forget the other factor the Car
itself.. ! performance Car, turbo cars or just a normal saloon Car
will affect the interval and how often you need to charge
oil.
I personally change my oil at (5000 km +-) for 5w30 Mobil One USA
Version. $16plus per bottle. I tried 5w 40 shell from 11k-19k
before i switch to 5w 30.
I am 80% driving on highway and per trip from home to work is abt
31km x2 for one more trip back to home. BUt i still change at abt
5k.
"Love your ride and she will treat you well"
