I changed the oil in my 1999 S320,M104,yesturday,using Lucas 10w40 oil I found on sale.That is supposed to be very good mineral oil,because they use the additives they are popular for in the oil treatments,anyway,it seems like the oil pressure is slightly lower with the new 10w40 oil,it used to stay a 3,top,all the time at idle,cold,warm,regardless,now it drops to 2 and 1/2,warm at idle...is the 10w40 too thin for the M104 engines?,now that we are going in the warm season,should I have bought 20/50 or 15/40?,also,what size is the tool for the rear end plug...looks like 1/2 in is a little small,I think its a 15mm...

You will surely get differing opinions on this but 20W50 is way too heavy for that engine, and your needle should not be at a constant 3 since, in my opinion, that is too stressful for the oil pump.

I would wager you also don't feel as much seat of the pants vibration like you used to with the heavier oil. If not, it is because your oil pump is no longer straining to pump the heavy oil.

Your readings are fine. Relax.

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2008 E350 4matic / Black/Anthracite

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Gone but not Forgotten:
2001 E430 4matic, 206,xxx miles, Black/Charcoal
1995 E320, 252,xxx miles, Black/Grey
1989 260E, 223,00 miles, Black/Black

92300SE, factory fill in the 104 engines was 15W40 mineral oil so I would recommend staying with that viscosity oil, but you shouldn't harm the engine with 10W40. As you have observed oil pressure will be slightly lower with lower viscosity oil. I use Mobil 1 15W50 in our 104 engines.

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Fred Hoelzle

i use 15-40 diesel in our 104

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1989 300ce 129k
( facelifted front,updated tail lights, lowered suspension,bilstein sports, lorinser front spoiler, MOMO steering wheel, remus exhaust,stainless steel brake lines). (Gone)

1997 s320 154k (what a ride). Sold with 179k miles. Replaced with Hyundai Equus

1994 e320 Cabriolet 108k



1972 280se 4.5 153k Owned for 12 yrs, sorry I sold it


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