Tough call not knowing more detail (model) about the players in question.
What you observe could be true, DVD-R (~1997) is older than DVD+R (~2002) so pre 2002 players will almost certainly be more compatible with DVD-Rs. DVD+R DL (~2003) and DVD-R DL (~2005) so the DVD+R DL could actually be more compatible with your older player crowd. (Above timeframes from my DVD Demystified book, a very good source of DVD history and technology)
Although not timeframe specific, this site
http://www.videohelp.com/dvdplayers can give one a good feel about user experiences with various DVD players and media playback experience.
DVD-R DL
426 DVD Players supports 60%
288 DVD Players do not support 40%
714 DVD Players tested (11041 DVD Players are not tested).
DVD+R DL
1041 DVD Players supports 75%
346 DVD Players do not support 25%
1387 DVD Players tested (10368 DVD Players are not tested).
I'd simply go with DVD+R DL and see how your crowd fairs. I use them all the time and have no issues. If you use LPCM for the audio vs Dolby when you burn the Disc or create a folder in PD11 this can also improve compatibility with older players.
Jeff