Many claim we should apply a 18 Lbf*ft torque to BMW cars.

As a matter of fact, BMW says two things about this, as shown below
The information is extracted from BMW TIS (Troubleshooting system).
Those figures apply to all BMW engines/models.

BMW states we should be using two different torque depending of the thread lenght, thus depending on the engine too. For instance those with a short thread are meant for engines that have a large cylinder head. Not our case on E46.

So it means we have to use the one that uses the long thread, in essence according to BMW, the torque to be used in our case is 30 ± 3Nm, thus 22 Lbf*ft, with a tolerance between 19.9 Lbf*ft and 24 Lbf*ft.

On the following picture you can see the plug on the far left is meant for large cylinder head, not our usual E46 plug. The one of the far right is the type we use on E46, notice the long thread here as opposed to the one on the far left, very short. The one on the left is also wider.

Finally the last picture shows you what/where the thread is, so we are clear on this.

Now I have taken into account that some people recommend to use Anti-Seize. As a matter of fact anti-seize products have a torque multiplying effect. But is is not recommended by BMW to use Anti-Seize when installaling plugs. Porsche also do not recommend using Anti Seize, along Pelicanpart and many other online resources. See below for some additional explanations.

Autolite:
We do not recommend the use of any anti seize products for installing spark plugs. Anti seize compounds are typically composed of metallic, electrically conductive ingredients. If anti seize compounds come in contact with the core nose of the plugs, it can lead to a misfire condition.
Anti seize compounds can also have a torque multiplying effect when installing plugs. This can lead to thread distortion and thread galling
resulting in cylinder head damage. Autolite spark plugs are nickel plated to resist the effects of corrosion and seizing. However, plug seizure is aggravated further when steel plugs are installed into aluminum cylinder
heads for a long period of time.

AC/Delco:
Do not use any type of anti-seize compound on spark plug threads. Doing this will decrease the amount of friction between the threads. The result of the lowered friction is that when the spark plug is torqued to the proper specification, the spark plug is turned too far into the cylinder head. This increases the likelihood of pulling or stripping the threads in the cylinder head. Over-tightening of a spark plug can cause stretching of the spark plug shell and could allow blowby to pass through the gasket seal between the shell and insulator. Over-tightening also results in extremely difficult removal.

If you decide to use Anti-Seize anyway, you should be reducing the torque value by 20%, therefore even the 18 Lbf*ft is not correct, well unless you decide to use the smallest possible BMW torque recommendation, 19.9 Nm

Importance of using the correct amount of torque
If a plug is not fully seated on the cylinder head, it will not transfer heat effectively and may leak combustion gases. Poor heat transfer can raise cylinder temperatures dramatically, causing detonation and plug or engine damage. Over-tightened plugs can also cause damage to the plug and to the engine head threads. Damage to the plug’s seals or the insulator can slow heat transfer and cause pre-ignition and detonation. Making sure to use the proper torque wrench will help to avoid these types of problems. Always install spark plugs in a cool engine, as hot metal expands and may limit the ability to tighten. Additionally, it helps to clean the threads in the head and on the plug.

Difference between Platinum and Copper sparks
The Platinum spark plug is the same as the Copper but the central electrode has a platinum tip, which improves corrosion and erosion resistance and provides longer life and smoother performance.
Many people do claim the Platinum type spark plug has a lower life expectancy than a classic spark plug with copper. Although the platinum one offers better performances, it is to be noted that they accumulate dirt more quickly than classic plugs. Therefore, a simple cleaning (Dry wipe/brush) will restore their full potential. Simply remove the plugs and give them a good wipe.

HTH

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