I'm happy to run conventional or semi-synthetic for 5k to 6k miles.
But I run thicker oils with Euro specs, so conventional 15W40 that is A3/B3 or 10W40 semi-synthetic that is A3/B4. In theory they should be good to 9k miles, according to my owners manual, and probably 10k miles for the semi-synthetics, but why push it.
Here (Oz) there is a big price difference between conventional, semi-synthetic and full synthetic. It's about double the price going from conventional to full synthetic. But I do tend to go longer with a better oil.
Here they have just released Castrol GTX UltraClean 15W40 which is a semi-synthetic but sold as the Castrol conventional product, it's rated API SN and ACEA A3/B3, and they have it on sale at $15 for 5L. Which is the most oil for my $ that I can get right now.
Right now I'm 4k miles into my last oil change with Penrite Vantage 10W40 semi-synthetic (SN, A3/B4, VW 502/505 with 1220 ppm Zinc and a starting TBN of 11) and it's looking fine. Not even thinking of changing it yet.