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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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I've been using Mobil1 75W90 synthetic hypoid gear oil in my transmissions manual ( ATF in auto), transfer cases, differentials and with the Ford additive in LSDs for years with great results. I always was concerned about one thing though.

The syncro's in a manual transmission operate with the use of friction. Would using a lubricant that significantly reduces friction in a system be detrimental to the proper operation of the syncros? Thus increasing the time for gears to match speeds.
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