BMW: bmw M3, 1998 sedan, bmw m3, syntetic

Question
1)  does rear differential fluid need to be changed?

2) how often does the radiator really need to be flushed?

3) synthetic oil needed on this model?  driving in southern florida during the summer, should I use a 15W oil or 10W oil?

thanks

Answer
The rear differential fluid and transmission fluid are rec. to be changed at 100k etc. but hardly anyone does it.  The radiator should be flushed at 100k for sure... not that hard of a job...there are coolant drain plugs both in the radiator and on the engine block. Syntetic is always nice but it is not needed... I am 90% sure BMW recomends 15/40 for that vehicle... I use Mobile 1 synthetic in mine. 1997 m3 sedan.