When I switched my E36 to synthetic, I implemented a reduced oil change interval at the beginning. I put in synthetic, and then I change the oil after 1500 mi, then after 3000 mi, and then went on regular changes every 6000 mi or 6 months. My reasoning was that this way I would ensure I flushed out all the dino oil and cleaned out the engine at the same time. I made that switch at 180k miles. The car has over 220k and the engine still purred like a kitten (and roared near redline) the last time I started it.
The ZHP has tasted nothing but synthetic since it was born.
Castrol Syntec 10W 40 every 10,000 irrespective of what the Si says, I did use VW full syn which was no problem and apparently lasts 30,000(changed at 10K) and I will say it virtually comes out the same colour it went in, I only changed due to the distance to my VW dealership as opposed to 1 Mile to my local parts store.
Is the VW synth oil just someone elses product in a different package?
Did you go through a change over process from Dino to Synth like Drz, or just let the system purge itself progressively by running it through the regular change schedule?
Synthetic oil is actually better than Dino oil, as it keeps your engine cleaner. The reason that your man has probably advised against it is that after running your car on Dino oil for a while, deposits build up around your seals. Switching to synthetic will clean away these deposits and often leads to oil leaks which are often (incorrectly) attributed to the usage of the synthetic oil. I have had no problems in switching to synthetic oil on well maintained engines with up to 100,000m on the clock. Although synthetic can provide extended oil change intervals, I would suggest you keep it at 5000m for the first 2 changes after the switch, as this will be when most of the crud gets purged from your engine.
My car had 109k mi on when I bought it with the only oil history I knew of was the dealer I bought it from said it had Valvoline High Mileage in it. 3 months later I changed to Castrol Syntec 5w-40 that I ran for 10 months and have since switched to Syntec 10w-40 at 1 yr intervals, about 6000 miles. No leaks or consumption at 123K.