Top up oil (GL4) in rear differential (with GL5 oil)

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Apr 25, 2022
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Hi,

I was working on my rear differential and when I was removing propeller flange from differential some oil (max 50-100ml which is 10% of diff capacity - 1L) leaked out. The vehicle was probably not 100% leveled.

In the diff there is this oil:
Castrol Syntrax Universal Plus 75W-90 which has API/GL5 specification

Unfortunately I don't have it but I have:
Castrol Syntrans Transaxle 75W-90 which has API/GL4 specification

After some reading GL5 means that it is for diff (extreme load with shocks) and GL4 is for transmission (not that extreme load). The main difference should be more additives in GL5 which provides the extra protection but these additives can be harmful in transmission so that's why GL4 is used in transmissions.

So I'm thinking:
1) do nothing
2) top up 100ml of Castrol Syntrans Transaxle (GL4) to the diff
3) buy 1 L of Castrol Syntrax (GL5) and use 100ml of it to top up the diff

And my question is: do you think that 2) is OK solution? Or even 1)?

I know that the best solution for the car is 3) but it is not exactly economical solution. So I decided to ask first before spending 20 USD on oil which will be 90% unused. It is 2007 E91 BMW.

Thanks
 
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Top off with any 75w-90 GL-5. No need to stick with Castrol. Don't use the GL-4 oil. It's not meant for Hypoid gears. You are making a bigger deal out of it than it actually is.
 

NiBe

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It is an economical question. I do have GL4 and in order to up with GL5 I need to buy GL5 (1L min) and then use its 10%. But I will believe you that I simply must go with GL5. Thanks. "you are making a bigger deal out of it than it actually is." what do you mean? That I should do nothing?
 

NiBe

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Ordered:
LIQUI MOLY Gear oil 75W-90 - 0,5 l GL-5

Thanks for help!
 

BlueOvalFitter

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Top off with any 75w-90 GL-5. No need to stick with Castrol. Don't use the GL-4 oil. It's not meant for Hypoid gears. You are making a bigger deal out of it than it actually is.
Shame on you Lubener. You should not be that rough on new members. I bet he won't ever post again!
J/K! :D
OP, Lubener gave you some good advice.
 
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I know that the best solution for the car is 3) but it is not exactly economical solution.
So I decided to ask first before spending 20 USD on oil which will be 90% unused. It is 2007 E91 BMW.
It is an economical question. I do have GL4 and in order to up with GL5 I need to buy GL5 (1L min) and then use its 10%.

It isn't economical to risk differential failure. 😜

Why should 90 % stay unused? You could use it later to change
your differential oil or even right now if you haven't already. It's
certainly a very good idea to swap the fluid from time to time.


Ordered:
LIQUI MOLY Gear oil 75W-90 - 0,5 l GL-5

Isn't it actually uneconomical to purchase such small quantities?
While I don't know how much you paid I do know 0.5 l bottles
commonly are much more than half of the price of a 1 l bottle.

However, glad to see you decided to top-off with some 75W-90
GL-5. (y)
.
 

NiBe

Thread starter
Joined
Apr 25, 2022
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It isn't economical to risk differential failure. 😜

Why should 90 % stay unused? You could use it later to change
your differential oil or even right now if you haven't already. It's
certainly a very good idea to swap the fluid from time to time.




Isn't it actually uneconomical to purchase such small quantities?
While I don't know how much you paid I do know 0.5 l bottles
commonly are much more than half of the price of a 1 l bottle.

However, glad to see you decided to top-off with some 75W-90
GL-5. (y)
.
In the diff there is few months old oil so I didn't want to change it. The top-off is done, I guess that it was not necessary as the oil was dripping from the filler hole aka the diff was full. I put 100 ml in, quickly close it and job done. Next year I will change the whole fluid.
 
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In the diff there is few months old oil so I didn't want to change it. The top-off is done, I guess that it was not necessary as the oil was dripping from the filler hole aka the diff was full. I put 100 ml in, quickly close it and job done. Next year I will change the whole fluid.
Wait it was already running out of the filler hole and you put in 100ml more?
 

CleanSump

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If the car was level and the fluid was running out, it was already overfull. No need for a top up.
 
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FYI:
I have a valvoline (this bottle doesn't say full synthetic or synthetic) GL5 gear oil without LS additive. It says on the back it's good for GL4 (and iirc non-synchronized) and GL5 gear oil changes ... and only for top up/off of the GL5 gear oils that requires limited-slip (LS) additive.

In general and typically there is a good chance for downward/backward compatibility with products but not the other way around.

my other bottles of Valvoline and SuperTech gear oils are GL5 "full synthetic" and both have LS additives. I have been using these in my xfr case and open rear diff (both require GL5 gear oil with no mention of LS additive requirements) with no issues for many years/miles. I found out that I had mistakenly been using gear oils with limited slip additives years later after carefully paying attention to the details. The bottles say GL5 so I used them and wasn't paying attention to "with limited slip additives" ... but according to experts, it should be ok and I have had no issues. Granted it won't work the other way around meaning if you have limited slip and use a GL5 without LS additives, It can/will cause chatter plus potentially other issues ...

However, follow your owner's manual.
 
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btw, valvoline does a good marketing job. I needed some DOT 3 brake fluid for an older car. DOT 4 is ok to use but I said why risk it, I'll just buy the DOT3 ... iirc, couldn't find one (dot 3) easily and DOT4 was popular in couple of stores.
Went to NAPA and saw valvoline "DOT 3 & 4" and bought it. Of course it's just DOT4 but the big letters " DOT 3" up front hooked me. lol
didn't care anyway since I like Valvoline but good marketing!
 
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Does your rear diff require gl4 which is synchromesh/yellow metal safe? Almost all limited slip diffs just require the LS additive and most of those are gl5 gear oils. There are some multi gear oils with both ratings but I don't see the big purpose in those. Better to just use separate fluids for separate components.
 
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Does your rear diff require gl4 which is synchromesh/yellow metal safe? Almost all limited slip diffs just require the LS additive and most of those are gl5 gear oils. There are some multi gear oils with both ratings but I don't see the big purpose in those. Better to just use separate fluids for separate components.
Hi,
are you saying that gl5 is not "synchromesh/yellow metal safe"?
Cheers,
Iain.
 
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Hi,
are you saying that gl5 is not "synchromesh/yellow metal safe"?
Cheers,
Iain.
gl5 is known for having that sulphur like smell from the high pressure additive which is good but that also makes it unsafe for yellow metals which are found in transmissions but usually not in differentials but some designs could have it. Combination gl4 and gl5 is typically just a beefed up gl4 that can provide protection without the high pressure additive. But the limited slip additive that rear diffs may call for is common in gl5 but not for gl4 which is typically for transmissions which doesn't need any.
 
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