With synthetic oils these days, the oil doesn't need to be changed until contamination limits are reached (carbon, fuel, water) or the additive package is "worn out". The base stock does not break down as readily as mineral oil does.
In an engine that does highway miles, it would not be unreasonable to expect the oil to still be up to task at double the factory specified interval for mineral based oil. It is still a good idea to change the oil filter at the recommended interval though.
I run full synthetic oil in my bike, which for the last two years has done a lot of commuting and short runs. I did a top end inspection recently and was surprised to find absolutely no sludging or carbon build up, looks like brand new inside. No wear on valve buckets either.
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