Can't say I know for sure if the synthetic increases the durability of your timing belt or not. I just had mine changed at about 96,000 km's which I believe is what is recommended and changed it regardless. Didn't even think about the synthetic oil and how it might help your timing belt.

Try asking some tech support hotline like Lube Tek from Chevron: 1 (800) LUBE TEK (582-3835).

I started using Texaco Havoline Synthetic 5W-30 at my first oil change and it's helped my engine greatly. Only go for service at 10 or 12,000 km's and it still purrs like a kitten four years later. Nothing needed except regular services although I stretch the interval.