Which DVDs to buy

Look in the Device Manager.  Click on the (+) next to DVD/CD-ROM drives. 
This will reveal the Brand and Model of drive.  Please post that.

Philips is a member of the DVD+RW alliance. Though the drive will take both -R and +R blank media, I would give the slight edge to using DVD+R.

Verbatim and Taiyo Yuden discs are the ones to buy.

I would beware of cheap discs.

Its Philips DVD+ RW SDVD8820

The Philips DVD8820 is a DVD±RW drive.  It should accept
DVD-R and DVD+R media.  The recommendation by ieee488 of
Verbatum and Taiyo Yuden is an excellent choice of media
brand.  For more info of media quality ..........

http://www.digitalfaq.com/reviews/dvd-media.htm