Conventional vs Synthetic plus viscosity, does it truely matter?

[Brxce]

Diff bushing is shot, so changing all 3 along with the diff cover. Obviously going to need to replace fluid. The owners manual states that except for AT (I’m assuming that means automatic) and nismo use genuine hypoid super gl-5 80w-90 or equivalent CONVENTIONAL (NON-SYNTHETIC) oil. I don’t think I’ve seen one post on here of anyone using 80W-90, and all the oils people used seem to be synthetic. I have a 6MT. I’ve also read that sport and base dif uses different oils as well due to the clutch disks in the VLSD certain fluid will break down the fluid that holds the disks together. Does anyone have any insight to this?

[Rusty]

In the VLSD. There is no clutch pack/disks in it. It's a sealed unit inside the carrier. Inside that is silicone fluid that is non-replaceable. Don't need a LSD additive either. a lot of people here use 75w-90 synthetic unless you have an OS Giken. Then you have to use their lube which is 80w-250.


DO NOT use GL-5 lube in the tranny. It eats the synchronizers. Use GL-4

https://www.widman.biz/uploads/Transaxle_oil.pdf

[Brxce]

Ahh i misunderstood what I was referring to then. The sealed viscous unit has those disks but they are in fact NOT clutch disks. I only have around 30k on the car and I’ve heard the trans fluid has a decent life span, and with the winter coming, I’ll probably hold off. The GLI-5 isn’t good for yellow metals, right? 75w-90 does seem to be the general consensus on here so I guess if everyone else is jumping off the bridge, I’m jumping too. Thanks, Rusty. I’m assuming both redline and motul 75w-90 are free from the LSD additive? I just looked on z1 and there was no mention of it on either product.

[Rusty]

Some gear lubes have the additive already in it. It's no big deal if you use it. It won't hurt a thing.

If you read the link I posted. You will know why not to use a GL-5 in the tranny.

In the LSD sealed unit. There is no clutches what so ever. There is 2 rotor vanes instead. The thick silicone fluid act like the disks. It's a nice smooth operation. You don't feel it working. The trouble with it is. When it gets hot. Say from doing trackdays. The fluid overheats and breaks down. It starts to act like an open diff then. Once cooled off. It will return back to normal for the most part. Also with mileage. The fluid breaks down. A lot of guys will swap the carrier out foe another type of LSD. Either a gear or clutch style.

[Brxce]

I don’t ever plan on tracking the car but I do daily it, and I’m a very spirited driver. With the stock lsd, would a diff cooler keep it cool enough to prevent it from acting like an open diff or is upgrading to say the giken or quaife or any of the other lsds? With oil temps up to 110-120 Celsius, i imagine all the other fluids get pretty warm as well

[Rusty]

Diff cooler will help. I have one. Plus the OS Giken diff. Diff cooler will set you back about a grand. I also have a diff temp gauge. It takes about 20 minutes to see the needle move off of 120F at 60 mph. The highest I have seen without the diff cooler on is 275F. The diff cooler will keep the temps around 175F or less. Depending on outside temp. My diff cooler is wired with 2 switches. One for the pump, and the other one for the fan.

[Brxce]

I haven’t checked anywhere but knowing the temps, and someone that’s got more than 8 months of seat time in a rear wheel drive and the behaviors of open dif vs lsd, I’m curious if anyones noticed the temp at which the stock lsd starts to act as an open diff. I live in New England so I don’t see the high ambient temps like someone in Florida or Texas, my diff may not get to 275 but I wouldn’t know. If the silicone in the viscous unit breaks down at 150 a cooler wouldn’t help prevent the open feel. If it breaks down at 250 and I never get above 235, the cooler would be a waste. At least in my head and the purpose of it.

[Rusty]

When I still had the VLSD in my Z. First started doing trackdays. It was fine at first. As I had more trackdays. It would start to fade away towards the end of 30 minute session. The more trackdays and the more mileage on it. The sooner it would fade. Towards the end. I could make it fade around the 15 minute mark. Just doing street. You should be fine for 50,000 miles at least.