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Gear Oil Questions

5570 Views 14 Replies 9 Participants Last post by  Don L.
Two questions, opinions and facts please (preferably facts!):

Okay, so I know the LSD takes Hypoid gear oil or regular with a friction modifier added.

I bought some Castrol Hypoy C today and the bottle says it's fine to use in LSD's and manual transmissions.

I mean, if it says it on the bottle I'm assuming it should be fine, wouldn't it be false advertising if it wasn't?

Secondly, I only bought 3 bottles (need about 4x1L bottles to fill LSD & transmission) because I had a bottle of Castrol SYNTEC Gear Oil kickin' around.

I would've bought 3 bottles of the SYNTEC, but it's about $15 a bottle where-as the Hypoy is $8, as much as I love my car I can't always afford the "best", and I put that in quotes because maybe synthetic is worse for our cars?

So, the SYNTEC, on the bottle, says it'll mix with conventional oils just fine. Again, I'm assuming it should be fine, wouldn't it be false advertising if it wasn't?

Now, while I'm sure everything will work, and there probably wouldn't be much noticeable difference between using regular gear oil in the transmission vs hypoid, or using synthetic all round, or mixing, I want to know the details. I've done some searching on the net and I have an idea on how it works and a few people mentioned it could be bad, but this was all speculation.

I want facts, smooth shifting and a LSD that chatters less. I'm probably going to do the change tomorrow either way, but I figured I'd post and see what CS thinks.
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What are the exact words that Castrol used on the back of the bottle that lead you to your conclusion? It may "work" in an LSD, but it may still need the friction modifier added.

I've always seen the "...for makeup of oil in limited slip differentials." - - In other words, it would need the friction modifier.
I'm glad you pointed that out, it's worth reading it again to be sure.

But....I don't think it says that, I just went out in the snow and grabbed the bottles from my car.

Let's read! (warning 8 megapixel photo!)


- Exceeds the service fill requirements of conventional and limited-slip differentials.

- Fully compatible with all conventional and synthetic API-GL5 gear oils.

Hypoy C:

- Meets the service fill requirements of conventional and limited slip differentials.

- Engineered for passenger cars and light trucks operating under high-stress conditions.
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Oh yeah, my friend keeps telling me to go get GM Syncromesh.. I've done some reading and it seems like mostly hype..
Okay, so it took about 2 hours to get the fill plug out..

Tried big ass wrench with extension, big ass socket, impact gun, putting a wrench on it and putting a jack under the wrench, shoving jack into wrench and then smashing it with some propane fire.

I have only driven it about 25 feet but the shifter feels loose and grindy? I hope it gets better with some time.

Haven't had the chance to do the diff yet.. Shrug.
Well, I can tell it was needed but there's not much improvement, but I can't expect a transmission with almost 300k to shift perfect every time (I rarely use the clutch, always shifts perfect when I do)
You rarely use the clutch pedal? Maybe BillyM's Shifter-Bushing Kit will help?
Yep, like never, well, as soon as the tranny warms up.. And sometimes if I'm driving hard just to be nice to the car or when I know I'm gonna fuck up the rev match. Really, I mostly just use it for drifting or sometimes racing to put some extra power down at the cost of clutch wear (hey, it's a racing clutch), or clutch braking (combining engine braking with clutch slip to slow the rear wheels more) but never shifting.. (PS: Don't try this at home, your drivetrain will hate you - mine does) :)

I was actually going to try the Marlin Crawler bushings, but I'll check those out too!

Do not use GM syncromesh,not enough viscosity for our older trans.
Yeah, I don't think I ever will.

Redline MT90 in the transmission and 75W90 with the limited slip is what I recommend. The MT90 stuff is like magic for Toyota transmissions. I had to put some generic synthetic gear oil like Castrol or Valvoline in my Corolla since it was the only thing local and it shifted like crap after using Redline.
I'll give it a shot, as I said I'm using the castrol and it's not that great.
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