In my experience... (I.e., please don't flame just because my experiences aren't the same as yours).
1. DVD-R is marginally more compatible than DVD+R. There are caveats, if your drives can do bitsetting (I think the BenQ can?) you can change the +R disc to read like a DVD-ROM (oversimplification, but in the software it's just a checkbox). People swear this works; in my experience, MOST DVD players will either play NO type of recordable media, or will play either. The few players I've seen choke on +R usually can start up the disc fine, but just have trouble continually reading the disc (like it's a reflectivity issue?).
It is possible the people you're sending discs to have units that simply won't work with recordable media. DVD players from the last couple of years should be fine, but once you get too much past that (4+ years) it's hit and miss.
2. I usually burn around 1/2 the rated speed of the disc and it works pretty well for compatibility. In my experience, 2X is a good speed to burn video DVDs. I've been doing 4X burning on some Taiyo Yuden 8X DVD-R media (Made in Japan Fuji) and they seem to be playing back well as well. I think it's part old wives tale and part true that slowing down the burn speed is more compatible. I wouldn't burn a disc at 16X for video at this stage for any reason (The media and burners are just too immature).