Thank you.
Timing guides are sort of like what is the best oil, everyone has their own opinion.
Here is what I learned from my 15 + years working on BMW's:
First of all, I've done the guides on my 540 so I know exactly wht's involved.
I've also learned this:
The guides can go at 100k miles, 150k miles or even 200k miles, it depends on too many factors to just say certain mileage is the key.
The keys to prolong their life (which I did) are pretty much universaly agreed on:
Lower the coolant temp. It simply makes sense. Less heat will make plastic less brittle.
Change oil often. Again, it makes sense. The better the lubrication property of oil, the less friciton between chain and guide surface.
Change outside TC tensioner. It also makes sense as if spring weakens enough it will cause the chain to momentarily slap on guide (or basicaly hit it) til the oil pressure takes over. We're talking very short period of time but if it's repeated enough...
Anyway, when I've done the VC gaskets recently I took a long time to inspect the guides with small mirror and physicaly moved them as I much as I could and I could not see any early signs of deteroation.