Not sure what type of differential oil to use front and rear on my truck? How do I know if I have a limited slip diff?

I do light towing under moderate conditions. My trailers never wiegh over 5000lbs. I live in oregon where lowest temps are in 40's. I have read threads about adding oil then slowly adding friction modifier then doing a figure 8 drive test to see if it hops around the corner, but, you only do this on limited slip diffs. I guess I just need to know how I find out what diff I have. My truck is bone stock as far as I know.

Thanks for your help.

Truck is:
manual
4wd
not sure of gear ratio

 

Manual calls for 75w-90 front and rear. It can be hard to find. I use 80w-90 front and rear.

The rear end (ratio) should be marked on a little tag attached to the rear diff cover. Its also marked on the build sheet uner the hood.

 

I've had real good luck with Lucas 80w-90. Works with limited slip diff's without additives and figure 8s. I've been running it since I got the truck at 100k, now have 210 and the LSD still works in the rear, although it is getting a bit weak. Both wheels still leave an even strip on dry pavement (gotta check every now and then ) but when one wheel is in a slippery condition only one will spin. Most I tow is 10k occasionally, 7k camper fairly regularly in the summer months and then the regular few hundred pounds of tools/gear. Good luck.

 

Since the diff oil doesn't need to be changed all that often and doesn't require that much I like to use high quality.
Royal Purple 75w90 works for me.

 

Amsoil for me.