Yeah, the 8000km mark is a pretty reasonable interval since Ford uses semi-synthetic oil now for their oil changes (So does the Mazda dealer where I take my car, same make and grade too --funny that...LOL). The really funny part is, I took my truck in for its first oil change the other day, and I had to almost "fight" with the service manager, since I only had 3000km on the odometer, and he kept saying , "Sir, you are not due for an oil change until 8000km or 6 months, and you have met neither requirement"... LOL... I told him "Yes, I know this, but, I never leave the "factory oil" in for very long"...
Sorry, call me "paranoid", but, to me, its a piece of mind thing...... I know with my new Mazda, I changed it at 1500km initially, and the oil was actually quite dirty... the second oil change at 6000km was much cleaner...so, that tells me allot. I won't even begin to tell you how often I change my bike's oil (every race weekend, and its VERY expensive), but, its "cheap" insurance.
My point is, the quality of the oil and filtration combined with the type of "work" you're doing with that engine will dictate the change interval. I find the 8k recommendation "spot on" given the sump size of the 5.4L engine (almost 7 litres) and the quality of the oil used. All of my VW TDI's (diesels) used high grade full synthetic (warranty requirement), but, lasted 16000km between changes (semi dry sump system with outboard cartridge filtration setup), and I had oil lab analysis done several times, and the results usually showed that oil would have easily lasted another 16000km's without issue. The old oil change "myths" of the 5000km change has been quashed in many articles and were only endorsed by oil companies to sell more oil. Now, having said that, I wouldn't recommend getting the $1.99/litre Kmart "special" discount oil and going 24,000km on it.....then, you are just asking for trouble..... LOL.
2010 XLT Screw 6.5 box XTR pkg, 5.4, bike hauler
2010 Mazda 3 Sport, Black mica, commuter duties
2008 Kawasaki ZX-6R - race ready (track duties)
2002 Honda Foreman 4X4, 26`ITPs, mud toy