Hi! muddy congratulations to your recent E38 acquisition. Mizhan has overated me as the sifu. I myself has just acquired a similar ride a couple of months ago and also begining to explore and learn more about this model.
To answer some of your questions;
1. Most Beemer owners of the 24V engine uses fully synthetic.
2. There aren't many E38 halfcuts around. So it would not be easy to find 2nd hand white lights. However I know someone who has a set of white tail lights. If interested just send me a PM
3. I am not into lowered suspension. I hear there is a shop Hokomotor or Speedzone in Batu Caves that carry stock for eibach & H&R. Not sure if they have any for E38.
4. Rims really depends on individual taste. Some stick to the standard 16" for the pure comfort while some goes up to 19 or 20" for the looks. If going this route be prepared to sacrifice quite a bit on the comfort including accelerated wear on your under carriage components. The 7 series runs on higher profile tyres so be prepared that the tyres will cost quite a bit too.
In my opinion the following looks nice
i) BMW Style 32
ii) BMW M Parallel Style 37
iii) The Alpinas
http://felgenkatalog.auto-treff.com/
5. Tinting - you need to be careful where you send them and the door panels will need to be removed. Some idiotic accessories/tinting shops do not have much knowledge about BMW and they can break your door panels and clips. I would suggest that you do it at the same time you refurbish your seats
6. Some members here have used Central Car cushion in Kepong. But although they are very near my house I have never used them. Maybe it's because I am used to or more comfortable with this guy in Puchong. He is an old timer with only 1 worker and his wife does the sewing. He runs a small business and don't have big dreams of making millions.
7. As for carpet mats, I think you can get them from Bav Auto or Munich Auto in Jalan Segambut. They are few doors from each other.
Hope that helps or you can send me a PM