Gear Oil Questions

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.