I'd follow the manual that came with your car over the Haynes when it comes to oil change interval--they over simplified it ignoring factors such as severe driving conditions as you have mentioned.
That said, the 5k to 10k oil change interval recommended by the manual has all but become politically driven to boast that their vehicle requires less maintainance than others in its class. In a world where more cars are being leased, one never truly sees the long-term effects of taking shortcuts on maintainance. So, I'd take the manufacturer's recommendations as minimum.
However, don't get me wrong. Synthetic is superior to regular oil when it comes to protection of your engine. I would switch to synthetic and change at 3k-4k interval until I can get an oil analysis to see how far I can take it, but the OEM filter should definitely be changed at 5k, at least.
And remember, you're not going to hurt your engine by changing its oil more often than you should. Possibly your time and money if you don't mind, that's why I suggest the oil analysis so you don't second guess.
Regards,
ed850
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