For 1998 A6 Avant Quattro C5 Audi recommends to use: API GL5, and SAE 90 transmission oils and genuine OEM transmission oil with a part number: G 052 145 A1. To select the right transmission oil see the table below.
How much transmission oil you need to put into 1998 A6 Avant Quattro depends on the engine installed, and some other conditions listed below.
Axle oil replacement in 1998 Audi A6 Avant Quattro might take 30–60 minutes, and will save you $30–$50 depends on the region you live in. The process is simple and requires minimum skills and time.
Unscrew the refill plug first then unscrew the drain plug. Use a spanner or a retched with the correct size head. When axle oil is completely drained screw the drain plug back with a new gasket. Complete the process by refilling the axle with new oil.
Please be noted that all capacities listed here are approximate. Check fluid levels when adding or refilling as recommended in your 1998 Audi A6 Avant Quattro user's manual. Keep in mind that all information here is provided “as is” without any warranty of any kind.
Before doing any DIY service please check your insurance policy and Audi warranty policy for your region. In some regions DIY service may invalidate your warranty.
DIY car service is not as difficult as it looks like and it might save you thousands of dollars per year. It might be even fun and a good thing to do with your kids on a weekend to teach them how basic machinery works. It's really hard to screw something up and damage your car permanently if you use right tools, follow Audi A6 Avant Quattro service manual and use right parts and fluids in prescribed amounts. Even modern cars stuffed with electronics can be serviced in your garage or even in your driveway. Changing engine oil, brake fluid, ATF, replacing brake pads, and etc. It's all may be done by yourself in a mater of a couple of hours.
Doing basic service for your 1998 Audi A6 Avant Quattro you're not only saving money, but also also improving your technical skills and learning something new.