How frequently should the oil be changed - 2008 BMW 535i Sedan

More than likely a bad oil pressure switch. However, if you have many miles on the car and did not change the oil frequently enough, you may have sludge buildup. Do not run the car with low oil pressure or you will blow the motor. It is worth the cost of assuming a bad oil pressure switch and simply replacing it.

oil change period is every 6 months or 5000 miles or 10000klms
however as the oil collects moisture from incomplete combustion any short runs that do not allow the engine to come to full operating temperature to boil off the moisture will need a more regular oil change
So if the car does runs of at least 1/2 hour the the 6 months will be ok
On the other hand if it is only used to go around the corner to a corner shop then more frequent oil changes will be required
Good policy is oil filter with the oil every 6 months or if more frequent with every second oil change

If you are changing seals and water pumps that frequently then the problem is run out in the end of the shaft. Could be a bad balancer or bearing. This is a major overhaul, I am sorry.

You should do this each 6 months or each 6000 km in severe conditions, which are one or more of......... frequent towing, all short-run driving, all stop-start driving, frequent dusty conditions or extreme heat.
For good conditions, none of the above, Toyota says you can stretch it to half the above frequency but I would not. Oil is cheaper than a new motor.
For the average driver doing a bit of all this I'd say every 6 months or 10000Km, and change the oil filter at the same time.

Change the oil. Car that don't go on long drives usually get moister in the oil from heating up and cooling allot if you drove long drives the oil would burn off the moister.In town driving car should have there oil changed more frequent then car that drive long trips.

With an older engine you need something of low viscosity when cold and high viscosity when hot and that is also resistant to mechanical shear forces. Try some branded (Mobil etc.) fully synthetic 5/40 multi-grade. Fully synthetic oils do not need to be changed nearly as often as the older traditional engine oils. Given the extended periods over which these oils can be used they possibly work out cheaper than using more frequently changed traditional (cheaper) oils. Always renew the oil filter when changing oil.

Unless you have a record of these fluids being changed in the past 18 months you should definitely change them. These fluids will degrade over time and need to be changed at appropriate intervals.
You should also flush the engine radiator and cooling system and replace with new coolant to the manufacturer's specs unless this has been done in the past 18 months.
Also ensure you change the engine oil and filter frequently (certainly more frequently than the manufacturer maximum interval specs which is based on optimum driving conditions which does not reflect reality.)

Frequent fluid changes = cheap preventative maintenance.

Brake fluid - every 2 years minimum
Clutch fluid - every 2 1/2 years minimum
PS fluid - every 3 years
Coolant flush and change - every 2 years
Engine oil - every 3500-4000 miles or 5 months minimum whichever is sooner
Oil filter - every second oil change.

Cheers
Sean
Volunteer contributor.

I'm sure that if you ask 10 different people, you will get many different answers. No different from any product with multiple brands. People have favorites based on past good history, referrals from friends or "experts" or whatever. The main thing is that you maintain your vehicle on a regular basis based on your driving habits - changing your oil every 3,000 to 7,500 miles depending your environment (dusty, dirty etc), do you drive in the city frequently, does the engine work hard/ frequently high rpms? bulk oil to synthetic, stay consistent with your oil change intervals.

any 15w/40 diesel oil is as good as another in todays world,toyota oil is one of the major companies oil anyway,if you stick to caltex esso shell ,bp any of the major oil producers you cant go wrong ,,even pensoil valvolene no worries

I cant help you solve your leak problem without more information... but... your oil will be black from a mixture of carbon deposits and acid. Some of this is from the blow-by when the fuel is ignited in the combustion chamber, some is from the oil breaking down when heated by the engines high tempurature. The more frequently you change the oil along with using very high quality oil (like a fully synthetic oil), the less carbon will be built up. Some people rarely change the engine oil, after a while (a few yrs of infrequent oil changes) the carbon will be substantial enough to blacken very recently changed oil.

The moral of the story, oil is cheap, engines aren't. Change your oil frequently it really does matter. IMHO