I am getting ready to go through and change all fluids over to red line synthetic.....Rwdlines web site said on the rear diff there would be two differnet weights on fluids depending on which rear diff i have........either Fords or a Dana how do i tell which one i have ??????

2002 F350 CC 7.3 ZF

 

SRW or DRW?
The SRW trucks have the Ford axle manufactured in Sterling so many refer to it as a Sterling axle.
The DRW trucks have the Dana 80 axle.

 

I have the SRW......That answers my question thank you

 

Glad to help.
FYI, the OEM fluid is synthetic already (at least in the rear axle). Ford says it doesn't ever need to be changed but many of us ignore that advice anyway. I change mine every 100k miles. If your diff is limited slip then be aware of which brand fluid you use because some synthetic oil already comes with the clutch additive while others do not.

 

There is also usually a couple of metal tags bolted to the diff cover. One of them will have the oil weight, and the other tells you the type of rear end you have. Sometimes you will only have the one that tells you which rear end you have. If it's a 4x4 there is a additive for the limited slip you're supposed to put in with the gear oil. Some oils will have it already in some type of additive in it.

 

I assumed that with the synthetic you didn't need the addictive. Am i wrong

 

You only need the additive if you have a limited slip unit.
Not all synthetic oil contains the additive, you will have to read the package or technical specifications for your brand of choice. If your brand doesn't contain the additive then add it in small doses until it feels right. Without additive a LS differential may chatter while steering the truck in a tight circle. Add a little (1/3 bottle) and try again. Repeat as necessary. Don't do what I did and toss the entire bottle in or you may end up overdosing on additive. Then you won't have any posi-traction at all. Which sucks whenever you try to scoot across an intersection quickly or driving in inclement weather.

 

The instructions from Ford says to add the whole bottle, and then top the diff off with oil. However I've read a lot of threads about the LIMITED SLIP not limiting slip very well.

The most common fix; Change the gear oil and use less additive.

 

I think that process will work fine if you are using Ford lubricants.
However if you are using Mobile 1 synthetic gear oil then you will convert your LS to open diff by pouring a whole bottle of additive in there.