Recommended Maintenance for the BMW E46 3 Series
Change your oil every 7500 miles even though the recommended interval is 15000 miles. This is not because the synthetic oil won’t last a long, but because the paper oil filters will disintegrate sending debris into your engine. I have always changed my oil with Mobil 1 5W-30 Synthetic Motor Oil. It is recommended to use the Mann-Filter HU 925/4 X Metal-Free Oil Filter to insure OEM parts are being used.
The air filter needs to be changed every 10k miles or 1 year. When chosing an air filter, make sure it is an OEM or of OEM standard air filter. Air filters that are not OEM can come with an oil sprayed on the filter designed to better trap contaminants. This is bad for these cars! It ruins the Mass Air Flow Sensor! A non-oil sprayed air filter that can be used is the Mann C3394 Air Filter.
Change your spark plugs every 50k miles even though BMW recommends every 100k miles. Stick with the OEM plugs made by NGK. The part number is NGK (3199) BKR6EQUP stock number 3199. These plugs are available at all major auto parts stores. DIY
The fuel filter is a commonly overlooked part to replace. Every 40k-50k miles the fuel filter should be replaced. The fuel pressure regulator is also part of the fuel filter. There are many different brands of fuel filters for this cars, in the past I have always used MAHLE Original KL 149 Fuel Filter.
In a rear-drive you have just one differential but in an XI you have 2 differentials and a transfer case that need to be serviced. Change this fluid every 30k miles, it is not lifetime fluid like BMW claims. In the differentials, a recommended fluid is Red Line 75W-90 Synthetic Gear Oil. Each differential will take about 2 quarts. For the transfer case, use Red Line MTL.
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Change this fluid every 30k miles. This is not lifetime fluid either! Redline MTL is the preferred choice.
Power Steering Fluid
One of the easiest things to do is flush your power steering system with automatic transmission fluid (ATF), which is recommended every 50k miles. The best procedure is the Turkey Baster Technique to flush the fluid.
Change your coolant every 2 years with BMW OEM Coolant mixed 50/50. Keeps your coolant system free of corrosion.
Do this prior to 100k miles and if you are still on the original one after 100k miles, don’t expect to be much longer. These OEM water pumps are mage of plastic and love to break and spill your coolant out in seconds.
Change your serpentine belts every 50k miles or if you visually see and cracking in the belt. This is a cheap and easy maintenance item, though can get a little tricky for Auto Tranny owners and the fan clutch. There part number for the water pump and power steering belt is Continental 6PK1538 and the air condition belt is Continental 5PK863. Every time I changed my serpentine belt, I had to also change my serpentine belt tensioner with Continental 49282 Tensioner Assembly.