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Discussion Starter · #1 · (Edited)
Hi guys,

Two weeks ago I've spent a Saturday to change the engine oil, differential and transmission fluid and the fuel filter (the tranny job included dropping the pan and changing the filter, cleaning, etc). Glad I did it since the differential "lifetime" oil was BLACK :yikes:, the tranny ATF a dark grey and the fuel filter was the original part.

Sadly the joy of running new stuff and the afterwards "smoothness" was shattered by a significant drop in my mpg. Before the change my average mpg city and highway driving combined was around 21.5. Today, 350 miles later it's only 18.6 :( I can definitely feel a slight "drag" when coasting compared to what it used to be before. Also before the change, when driving in straight line and no incline, giving gas only now and then was all that was needed to maintain a constant speed. But now ... I have to give it gas all the time :tsk: (which would explain the lousy mpg).

So here are my primary suspects:

1. Engine oil: have put in Mobil 1 0W-40. Although I used it with great results on my former 325 and it cleans the engine wonderfully, I think it's a little too thick for my car. I'll leave it in until I reach 3000 miles on it then I'll change it with German Castrol 0w-30.

2. Differential fluid: used RedLine 75W-90 Gear Oil and it was MUCH thicker than what it came out. I plan on changing it with Amsoil Extreme Gear 75w-90. According to published specs it is a little thinner at lower temperatures (I doubt the diff reaches 100 Celsius in normal driving).

3. Tranny ATF: that's a tough one to decide. Have put in Castrol Import Multi-Vehicle ATF. I will do a second drain and filter change in 700 miles and now I am wondering whether I should not go with Amsoil Import Multi-Vehicle or Valvoline Import ATF for second change since, if I remember the specs correctly, they both are just a tad thinner. But on the other hand I am reluctant to run a mixture 3 ATFs on my tranny (old, Castrol and Amsoil/Valvoline).

So any thumbs up/cons to my plan ? Have you had/seen this issue before ?

Thank you.
 

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Discussion Starter · #5 ·
Good question z_driver ! I drove 350 miles since the filter change, would that be not enough for ECU to get it right/adjust itself ?

Also, how would wrong data in the ECU explain the "drag" I now feel ? Why it "rolled" so much easier before than now ? Will that not have something to do with increased friction ?

When I attempted to use 20w-50 oil in the past on my 325i, the mpg plummeted right away. Using a thicker oil than recommended does have a negative impact on your mpg. Likewise, although RedLine and Amsoil are the same weight, the viscosity of Amsoil is slightly less than RedLine's therefore I would expect it to have some impact my mpg.
 

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25,378 Posts
The OP might find the original threads on this topic in one of these which I found by typing /mpg F3 into the bestlinks:

- What gasoline mileage should an E39 get (0) (1) (2) (3) (4) (5) (6) (7) & the truth about mpg calculation accuracy (1) (2) & E39 great mpg stories (1) (2) (3) & your basic E39 awful fuel mileage issues (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25) (26) (27) (28) (29) (30) (31).
 

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Discussion Starter · #7 ·
Hi all,

Have not reset the ECU yet - I'll wait for the weekend due to the 20 min idling "afterwards" requirement ... But I have my doubts that that will solve anything.

So as the first step I've decided to drain the RedLine Gear Lube 75w-90 and replace it with something else. So here are the candidates:

- Differential (conventional, i.e., non-limited-slip E39s): The Bentleys specify "BMW SAF-XO synthetic final drive gear oil" which doesn't exist (it's Castrol). The Bentleys don't specify the weight (it's SAE 75W-90); and the Bentleys don't specify the quality (it's API GL-5 hypoid gear oil). Given that, suitable replacements are Amsoil Synthetic Manual Transmission Fluid (MTF) API 75W90,Mobil1, Royal Purple Max-Gear, & Redline SAE 75W90 API GL-5 synthetic gear oils. Unfortunately, there's no way to tell which of the three (3) different differentials you have w/o looking at the numbers molded into the metal. Bentley page 020-11 & 020-26 & 331-6. [Volume: Type G=1.7 quarts, Type 188 Compact=1.1 quarts, Type 220/215 Compact=1.5 quarts.] Replacement Interval: Lifetime fluid.
Now something is not clear to me here: Amsoil Synthetic Manual Transmission Fluid (aka the "MTG") is an API-GL4 oil. I doubt it has any friction modifiers and because of that I believe it is the thinnest of all oils above. So my question is why it is mentioned there when AMSOIL lists it as GL-4 oil ?

I've put together a list below with all candidates and their viscosity at 40 and 100 grade Celsius. As previously said, Amsoil Synthetic Manual Transmission Fluid is the thinnest (likely no friction modifiers) while Royal Purple Max Gear is the thickest (likely lots of friction modifiers).

Since we have non-slip-diffs, what is the impact of friction modifiers to our mpg ?

cSt 40C
=========
AMSOIL Synthetic Manual Transmission Fluid (MTG): 88.4
RedLine NS (non limited slip diff) Gear Oil: 95
Castrol Syntec Gear Oil: ??? (not published)
Mobil1 Gear Oil: 99
AMSOIL Severe Gear (SVG): 109
RedLine Gear Lube: 115
Royal Purple Max Gear: 119

Cst 100C

AMSOIL Synthetic Manual Transmission Fluid (MTG): 14.8
Castrol Syntec Gear Oil: 15
Mobil1 Gear Oil: 15.2
RedLine NS (non limited slip diff) Gear Oill: 15.6
RedLine Gear Lube: 16.4
AMSOIL Severe Gear (SVG): 16.8
Royal Purple Max Gear: 17.5

So should we go with fluids with no friction modifiers or brew a a mixture of two ?
 
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