They're called "crush washers".
To change the oil you need:
1) Oil filter
2) Crush washer (if available)
3) 4 quarts of oil (3.5 to be exact)
4) 10mm wrench/socket to remove the 6 bolts on the stock "skidplate" (LOTS of people have snapped the heads off of those bolts - be careful)
5) Socket/wrench for the drain plug (14mm? I forget)
6) Something to catch the old oil in
7) A bunch of paper towels
8) A (clean) funnel is nice - but not required
My super-anal procedure (hey, I'm an engineer :p ):
1) Pop the hood, remove the oil fill cap, then get under the front end and start taking off any necessary skidplates etc. Position a catch pan under the oil pan (not to be confused with the transmission drain pan. The tranny pan is further back and is large and flat. The oil pan is smaller and further forward).
2) Remove the drain plug and watch the oil start dumping out (you will get a nice splash of it on your hands... have a couple of paper towels (or better yet, those blue shop towels... they rock!). It goes a bit better if the truck is warm (oil will flow out a little better - but not required... 'course you're more likely to burn yourself) and if it is not windy (oil will get everywhere as it drains).
3) After oil has stopped flowing from the pan (there is always a little stream still trickling out... at some point you have to call it "good enough"), wipe off the drain plug, put a new crush washer (if available) on it (remembering to remove the old one first and note which way it goes on), and put the plug back in place on the oil pan. Tighten it down until it is good and snug - but not too tight (I forget the torque spec... it's not very important anyway - it's not under any pressure or anything.... the idea is just to get it tight enough so it won't back out or leak but not so tight that you start stretching threads or damaging anything)
4) Move your catch pan forward so that it is underneath the oil filter.
5) Grab the oil filter and unscrew it by hand (just like any other screw). If someone else put it on, it may require an oil filter wrench - but I've always been able to remove mine by hand. As you remove it, oil will start to come out around it... try not to make a mess
. Dump the contents into your catch pan, drop the whole filter in there... whatever.
6) Use a paper towel or two to clean off the mount where the filter goes... particularly the shiny ring where the rubber o-ring on the new filter will make contact.
7) Get your new filter and a quart of oil. Start filling the filter up with oil (this takes awhile - you fill it up and it starts being absorbed... and then you put in a little more... and then a little more etc etc etc) Overall a Nissan filter will take nearly a half quart of oil. Dip your (clean) finger into the new oil and rub it around the o-ring on the new filter to get a coat of oil on it.
8) Screw the new filter onto the filter mount (being careful not to dump the oil that you just put in it everywhere). It will screw on very easily and at some point you will be able to feel that the o-ring has made contact with the surface that it mates to. Once it has made contact, tighten the filter another 1/2 to 3/4 of a turn (Once it makes contact, I note where my thumb is on the filter - and then turn it until my thumb is 180 degrees (or a little more) from where it was)
9) Get up, and start pouring oil into the oil fill tube in the engine compartment (on the passenger side of the engine). The funnel comes in handy here. It takes *exactly* 3.5 quarts. I don't even bother with that pour-a-little and then check it and then pour a little more garbage anymore. I just dump in 3.5.
10) Check it with the dipstick to make sure there is *something* in there - i.e. you didn't forget to put the drain plug back in or something (don't bother adjusting the level right now.. you need to start and let the oil cycle through the engine and filter a couple of times before you can get a true level. The dip stick on these trucks is awful... you have to look at all four sides of the tip and try to guess the correct level (generally you go by the side that is the lowest). It is also difficult to see new oil because it is so clear. Put the oil fill cap back on.
11) Start the truck up and let it run for 30 seconds or a minute.. Get out and check under the truck to make sure the oil filter and drain plug aren't leaking
12) Stop the truck, wait a minute or two, and recheck the oil a few times to make sure it is at the right level (halfway on the dipstick). Add more oil if necessary (remember, though, that too much oil can be just as bad as too little.. but in general if you keep it within the limits of the dipstick you won't have any trouble).
13) Close the hood, put the skidplate back on, clean up, dump the used oil at Auto Zone, and you're done