This is not intended to be the undisputed code of how to care/clean/polish your 928, this is just my limited experience and what works best for me. A good starting point for those green behind the ears would be Larry Reynolds articles which have far more knowledge and experience than I could ever hope/want to achieve in the car cleaning world.
On another note I don't have infinite time available to do this stuff, a) I have two demanding children and b) any spare time I get I like to drive it :-).
[Note: I'll add some photos later]
Before I came to this side of the water I had a black 2.8i Special, one of the last to be made, anyway in 5 years of ownership never got the car to polish up the way I wanted. So when I purchased my 928 I vowed that there must be a better way, found Larry's page and read through his articles, ended up purchasing the Zymol products.
Clear Auto Bathe, HD Cleanse and Carbon Wax all arrived in the mail and I couldn't wait to get started. Unpackaged a bunch of towels I purchased from Costco and began working on the car, after washing I proceeded with the Cleanse and was surprised at the chocolate aroma but otherwise the experience was fairly uneventful although the color definitely had more depth to it.
On to the wax, this stuff is made out of natural ingredients so has to be kept in the fridge (surprisingly the wife didn't object) to prevent from going 'off'. Anyway this stuff went on OK but was a bitch to polish out, when I finally got it looking right a month later it was all for naught as there was no wax left on the paint !!!! Now I know it isn't meant to last as long as other waxes but I expect it to last longer than 2 washes or so. At this point I was a little disheartened.
When the car was parked in the garage under the florescent light or in direct sunlight I could see very fine scratches which wasn't quite as good as I wanted although it was much better than having swirl marks. For the next few months I just used the same liquids I do on the family Jeep and Van, the three step Maguiars, Clean polish and Wax stuff you can purchase from PepBoys for a reasonable $$$$. Although the finish wasn't as good in terms of depth it at least lasted a couple of months no sweat, still had those fine scratches though.
After reading Larry's then updated articles he mentioned that some cotton towels actually have polyester in them which can lead to fine spider web type scratches on the paintwork, hmmmm. So I ordered some towels from him, some 3M Imperial hand Glaze and some One Grand Blitz Wax, his towels were waaaaaaay softer than mine so I donated the Costco ones to my wife, kind soul that I am.
3M imperial hand glaze is not perceptibly better/worse than the cleanse but you get much more for your $$, the real improvement came with the better towels as now those fine scratches were gone. Application is easy, buffing out can take time, this stuff must have a bunch of oils as when it comes into contact with water (left over from cleaning in those hard to dry areas) it does a good job of smearing. My advice is to take your time and use plenty of towels.
To avoid the smearing as much as possible I removed the rear light lenses, front and rear license plates and the little bumperettes on the back.
So to my neighbors and family I am a little weird as I wash my car in all weathers and I do it by hand !!!, but as for applying wax with your fingers this was pushing it a little far....... but surprise it works. The Blitz waz goes on well but it takes time with your fingers which gives you time to spot all those imperfections. But the best part is that it comes off easily, use one towel to get rid of the excess and the other for buffing. What a difference to the Zymol, again I couldn't detect a big delta over the Carbon Wax but it looks like it will last and it doesn't take forever to buff out or cost so much $$$$.
So I now have the desired finish and some products I am pleased with, nothing like a deep clean black finish on a car.
One thing I am not keen on is glossy interior surfaces that came from the manufacturer matt, while there is a need to protect the surface from UV (especially in the 928) I see no reason to want to see my reflection in the dash for example. While at the dealer they had obviously used plenty of the 'Armor All' like cleaner and the first thing I did was to clean it off, baby wipes work wonders for this and for cleaning various interior surfaces.
So far I have been using the Vinylex by Lexol, if sprayed on you get the said gloss but if you spray onto a cloth and gently rub you get a much better (IMHO) look. Even so it is still a little too glossy, so I ordered a bottle of 'Something Else' to see if this was any better, I am now a happy camper.
Now you can put plenty of this stuff on without worrying too much about the shine factor, however, its number one asset is the consistency of the finish. It leaves all the surface looking the same which gives it the thumbs up in my book.
So on my 2.8i the leather was in need of replenishment and did well when cleaned and fed, that was conolly leather and as the Germans use a different breed of cow (sane ones ???) I went with Larry's suggestion of Lexol cleaner and conditioner.
I had the same problem before with cleaner in that where you have very fine cracks the cleaner has a tendency to remove a little of the colored leather (which was probably loose), if you have black leather this is less of an issue. To the trained eye (i.e owner) this looks like dirt in the cracks but don't be tempted to scrub with more cleaner to make it better, luckily I didn't make this mistake in the 928 .....
I find the cleaner really works well on the steering wheel, all those sweaty palms correcting oversteer ??? more likely gunk form the mechanic .......
Conditioner works fine too, make sure you remove the excess as it leaves the surface a little tacky like someone has smeared your seat with vasoline or something. I tend to do the drivers seat every 3/4 months, as the other seats are used less often once a year seems OK to me.