Hey guys,

I had a couple questions regarding synthetic oil.

First of all, since the rear wheel bearings will be replaced, this is a good time to replace the diffy fluid too. So I was thinking about getting some valvoline full synthetic gear oil. How much gear oil does the diffy hold? 3 quarts? 4 quarts? And will full synthetic make it leak?

I also had a similar question for an oil change. I'll be changing my oil at the same time. I had been running a synthetic blend but I saw a good deal on full synthetic. The deal is valvoline synpower full synthetic oil and a mobil 1 oil filter for $22 after rebate. Will full synthetic oil make leaks happen here too?

 

I dont think it will, my dad switched to synthetic last year and he hasnt seen any leaks, Im not sure about the differential but Im sure it wont leak. and I think it holds 3 qts but dont quote me on that, its just a guess,lol

 

No it wont leak if it doesnt now. .. Amend that to say if you do a Seafoam crankcase treatment and it doesnt leak after that then it wont with Synth either.
If it DOES start leaking, then just add about 1/2 treatment of Lucas to swell the seals and gaskets back.

 

In my experience if the car is a bit older and has higher mileage anything over 75k IMO I would stick with the blend rather than fully switching over, (the oil only not sure about the differential oil) I found that some vehicles tend leak due to stress cracking after switching unless done at a lower mileage or when you get your first oil change when the car is new.

The synthetic blend is only at max 30% synthetic and thats at its max depending on what brand you use, some are only 5% synthetic so keep that in mind.

as I said IMO I would stick with the synthetic blend for your motor oil depending on the mileage and condition.

 

Synthetics causing leaks is a myth, if anything they'll clean sludge and cause an already failed gasket to leak.

Also if you read bobistheoilguy.com there's some great info there. He explains the difference between dino/synthetic (other than the obvious) is that a 10W-30 dino oil is based on a 10 weight base, whereas a synthetic 10W-30 is based on a 30 weight. That's just one of the major differences......

 

And that ...ahem... is a load of crap. IMO
Mike 98 got it right

Nothing personal... but ONE guy's experience doesnt matter to a hill of beans.. even if it does, I switched my last Tbird to Mobil1 from pennzoil {though religiously serviced .. whew!}..when I bought it at 143,000 miles.. drove it another 150k or so and only time it leaked was when I had a bad oil filter seal.
So I guess MY experience trumps yours... except I've used M1 in about 5 different high mileage cars over a period of 20 years. Without any seepage leaks.

I guess I could be less blunt about it but there was a raging debate on this for a couple weeks on the old Mustang newsgroup and it turned out that a couple guys had bad old style cork gaskets and all the others just heard it from their dad.. or uncle or a guy who knew a guy who was an engineer.

 

Thanks for the help! I'm going to get my full synthetic oil and filter either later today or tomorrow and change it.

I had 1 more question. Do you know how much gear oil the rear diffy takes? 3 quarts maybe? I was trying to see how much gear oil I needed to change out the fluid in the rear. And also, what about the transfer case? auto tranny fluid for that? How much?

 

On the rear-end, I just fill it till its level with the hole you pour it in. It'll drip out any excess, once it stops just plug it up and your good to go.
Edit: I used Royal Purple Oil for the rear-end if it means anything