One detail I wanted to point out (although it is probably all over this site) is that the oil filter housing (or center bolt) has that 'stem' on the bottom with the o-ring to hold oil in the top of the engine, so you never start the engine with dry cam bearings.
The down side is, if you dump the oil
and then pull the filter, the volume of dirty oil in the top of the engine and filter (about a quart) will drain to the pan and mix with your fresh oil.
So, always pull the filter top first, and then dump the pan. Also, do it while the engine is hot (right after driving it ideally), as it flows better/faster but in addition the impurities will be in suspension in the old oil and drawn out with it.
I do not advocate Marvel Mystery oil in regular oil changes. I use it to lubricate air tools and free frozen cylinders only. If you feel the need to 'clean' your engine internally, put a new filter in with a suitable amount of the Marvel oil (or I prefer Rislone for this, but I have no idea if it is still made??), but only run the engine a few minutes with this in there, dump this oil and re-change the filter to a fresh one and refill with regular oil only.
No big difference between synthetic or dino oil in these cars with some mileage on them. If you drive year round, mind the temperature ratings and suggested weights in the owners manual, and you may want/need to change weights with the winter/summer seasons, particularly in the northern regions.
I only drive my ti in the warmer months, and I run Mobil-1 15-50 (NOTE: I have an S52 engine and I don't think you would want this heavy an oil in an M42/M44 engine). If I did drive in the winter, I would switch to 5-40 or 5-30 for the colder months I suspect.
BimmerBum had a deal recently on magnetic oil pan drain plugs and I always suggest installing these at your next oil change.