SLK55 Differential Oil: Replacing the differential fluid. The below from a friend who is a Master M-B technician. I found the fluid he speced at ECS for $24 a 1/2L you will need 3 of these bottles. Either look up using M-B p/n or the Castrol name.. I think I paid $77 delivered.
Mfg Part # 001989520310 ECS Part #ES
"Just as I guessed, it uses some specialized fluid. The correct rear axle oil for your vehicle MUST meet the specification “MB 235.61“
You will need 1.0- 1.3 Liters of the oil.
The easiest to find oil that meets this specification is “Castrol Syntrax Limited Slip SAE 75W-140” it is available in various places on the inter-webs. It is also sold at Mercedes-Benz dealers in 0.5 L containers with the part number 001 989 52 03 10.
The container must have clearly notated on it somewhere that it meets “MB 235.61“ and it has to be particularly expensive.
If it lacks either of those characteristics, it is not the correct fluid..."
SLK55 Engine Oil: Mercedes Specs Mobil-1 Euro spec 0W-40W. They built the motor, I would go with that. Besides I can get it at ANY Walmart (do they have Walmart's in the UK?)
SLK55 Oil Filter: The SLK55 uses a shorter oil filter than the r113 engine used in the Larger Class M-B, due to limited hood space.
The proper filter is a MANN FILTER HU 715/6x wool filter... again I bought at ECS ES#2631385 P/N0001802809 It includes the 4 o-rings needed.
NOTE SLK55 Oil Change. There are 2 sump drains. Meaning you have to get under the car, remove both engine covers to access the forward and rear sump drain plugs. You cannot do a complete oil change using a suction pump from above the engine.
SLK55: Spark plugs. Do on a slightly warm engine, It's winter here (Jan 2019) engine should be about 100F Removing the plugs: remove the wire and boot, turn CCW the plug about 1/2 turn. Use High Pressure air around both plugs to get rid of leaves, dirt, dead mice, etc so it doesn't fall into the cylinder. Also if the new plug is chromed/silver around the threads do not use anti-sieze, it will not torque correctly and could strip out the head. (this from plug manufacturer.) They will remove without issue when you change then again. I can't remember exactly which plugs I used but the were Iridium's
SLK55 Air Filter: I bought MANN C 3698/3-2 Air Filter Element from Amazon for $20.00
I hope this helps...
Next to do: replace both Diveshaft Flex Disks and driveshaft support bearing.