There is a thread in the sticky section regarding how to do this, and while the diagrams from Vadis are helpful, they don't tell the entire story. There is a special tool required to open the fill bolt on the M66 transmission if you don't feel like dropping the sub frame. I just completed this oil change last night, and thought I'd share some pictures from the process in the order I did them.
1: Drive the bjeebers out of the car to get the transmission up to temperature.
2: Jack the car up on 4 jack stands. Make sure the car is level as this affects the fill amount. I used a 2' bubble level placed on the oil pan to make sure I had the car truly level. If there is a better place to level off of, someone please speak up.
3: Pull off the drivers side front wheel.
4: Locate the fill bolt and remove it first. After being on the transmission for 6 years, mine was on extremely tight. I needed penetrating oil and a lot of muscle to get it loose. Here are two pictures, one of the location of the fill bolt, and the other of the Volvo tool necessary to get it off.
5: In my case, I think the transmission was over full because the moment I pulled the fill plug used oil began to drain out. In anticipation of this, I had my oil catch pan already in place, but be aware that while your driveway will stay clean, your sub frame will not and you'll need some rags or paper towels to clean it up.
6: After opening up the fill plug, time to get the drain bolt off. You can use the special Volvo tool, but a 24mm socket (15/16")will also work. I could not put enough torque on the bolt to get it loose, even after 2 hours of soaking in penetrating oil. I had to hit it with the impact wrench to get it off. While the oil was draining, I took a sample to send to Blackstone Labs for analysis. Along with some fresh oil to test, I should be able to determine how much wear is occurring in my transmission and a general idea of the change interval we should be observing. I have 93,000 miles on my car at present and will report back when I get the test results. Looking in the hole with a flash light, you can see some of the gear teeth and it isn't a bad idea to turn the wheel hub which will turn the gears and allow a basic visual inspection of the teeth. Here is a picture of the drain bolt location and the bolt off for draining and inspection.
7: After letting the transmission drain, time to put the plug back in with a new crush washer, or with a replacement plug. I let my transmission drain for 3 hours to make sure every last bit of oil that could get out did. After all, I'm only going to do this once every 100K miles or so. I chose to replace the drain plug with a magnetic plug from Viva Performance. This plug goes back in a little easier as the socket size needed is 17mm, not 24mm. The torque spec I was able to find was 35Nm. If this is in error, someone with access to Vadis please correct me.
8: When you've got the drain bolt back in and torqued, time to fill the transmission. I use an inexpensive oil transfer pump to get the oil from the containers into the fill port. A funnel and hose would work fine as well since this is very thin oil. I found that with the hose in the fill port, it is a little difficult to tell when the transmission is full. After pumping in two full liters, I pulled out the fill hose and some oil began to drain back out. There is nothing wrong with this because the proper fill amount is when the oil is level with the lower threads on the fill port. Letting a little oil drain itself back out will ensure a proper fill is achieved, but this is predicated upon the transmission being level to begin with. Here is a picture of the fill port once the oil level stabilized.
9: Time to put the fill plug back in. Getting the threads started was a little tricky working around the sub frame member, but once in, you can use your finger tips to spin the plug till it is in reasonably snug. I believe the torque spec for this plug is the same as the drain, 35Nm. This is a little harder to torque because of the special tool needed, but with some extensions and adapters it can be done. I needed to use a 3/8" drive torque wrench adapted up to the 1/2" drive of the Volvo tool because most 1/2" torque wrenches can't be set low enough for 35Nm. Here is a picture of the frankenstein rig I ended up with to do the job.
10: Once everything is tight, clean all excess oil up so you can check for leaks, put the wheel back on, and take the car for a test drive. After the test drive, check and make sure you've got no leaks.
While I have very little doubt that most of what I experienced was placebo effect, I thought that the shifting was smoother and the transmission quieter. I especially thought I noticed a difference when rev matching a 3-2 down shift in that there was a noticeable smoothness to the engagement of the gears and it felt like the engine revved a bit faster during the blip in neutral. All of that to say, the car shifted smoothly and quietly before, and any improvement I might have made is minuscule. The whole point of changing the oil is to try and make the transmission last as long as it can.
In case you're wondering, the tool to buy is part number 9997301 from SPX/OTC USA (http://volvocars.spx.com/). Thanks to pczeilon for providing that information. I paid $64 with shipping for this tool and received it a few weeks after ordering it. here is a picture of the tool.