No personal experience with Royal Purple yet. Have had nothing but positive experiences with AMSoil... not just the products but also getting information from them. Last time I wrote them I got a response the next morning.

Before finding out about AMSoil I used Mobil 1 religiously... it's OK. Tried Redline too and that was about the same. Both are good.

You're making oil changes far too complicated for yourself. Check your VX's owner's manual for factory recommended service intervals and service items. If you don't have a manual I think the manual package can be purchased from any Isuzu dealer for around $30 to $35.

Different brands have different additives. Mixing brands may cause unwanted chemical recations (unlikely). But to be safe, it's best to stick to one brand... if you do have to mix temporarily, I don't think it would cause any long term harm.