Reading the repair manuel and it says to change the o-ring on the diff when you change the fluid. Is this neccessary or can i use the old one and be fine????

 

First, what year, model and mileage?
Have you ever done something like this before, what gear oil are you using?

 

2001 disco II le...102,000 miles...I dont do any off roading so im using 75-90 gear oil....my cousin is a mechanic si hes helping me....shouldn't be too tough

 

So, on a D2, the fill plug is plastic, prone to breaking, and really should be replaced with one of the solid metal or brass plugs.
As for the oil, consider going heavier, to the 75W/140 for extra wear protection for your spider gears.
Also check each drain plug for metal chunks on the magnet.
Do the same thing with the t/case, using the same oil and inspection, 7 quarts in all.

 

Is there any downside to using the heavier gear oil in discos? I've been using the 75W90.

 

No downside I can see, everything runs quieter, less metal mud on the drain plugs so less wear and the cold here in Colorado sure doesn't seem to be an issue. The t/case has the biggest change, much quieter.

 

harder to pump out, depending on what method you use to get the gear oil into the diffs./T-Case...

 

where can i pick up the plugs and is there an o-ring i should buy also.....

 

Well I only have changing the differentials and transfer case oil left to do on my list. I will try the heavier oil this time.

 

expedition exchange sells the brass plugs. i got a couple of them because i am a neanderthal and shredded the stock plastic one on my DII. they are worth there weight in gold in my opinion. oh and they come with the O-ring on there.