Migh Mileage - Maintenance Advice requested

[scirocco]

This is about my 2001 X5 with a 3.0L inline 6 cyl engine.

The truck has more than 200K miles and keeps running and running sweetly and reliably. Rock solid. It is my daily driver and takes a lot of abuse, I will say. I pile up my tools in it and do field work such as HVAC, Plumbing and other Handyman type of work.

But I worry about that other maintenance should I do considering the high miles.

Should I change / replace the Timing Chain guides? I know that is critical for the V8's but this is an inline 6. If yes, how difficult is it to do DIY on it.

Transmission - I will NOT replace the fluid. Should I top it up? What top up flid should I use.