So far, I've seen results with these DVD-RAM media.
- Hitachi-Maxell 5x DVD-RAM 4.7GB
- Hitachi-Maxell 3x DVD-RAM 4.7GB
- Matsushita-Panasonic 3x DVD-RAM 4.7GB
- Lead Data DVD-RAM 5.2GB (2.6GB per side)
At least, Maxell 3x, Maxell 5x, and Panasonic 3x seem to work reliabiy.
Panasonic (made by Matsushita) 3x DVD-RAM on GSA-4120B tested by Mr. C. in Seoul (who a few minutes from Yongsan market.):
http://www.storageinfo.co.kr/EzMODse/viewtopic.php?t=28283

Media Information
Region information N/A not a DVD-VIDEO
MediaCode MatsushitaElectricIndustrialCO.,LTD.M01J3004
Format Type UDF 2.84
Volume Name
Application id
Implementation id
Recording Date/Time (mm/dd/yyyy) 0/ 0/ 0 0:00:00
Format Capacity 4.27GB(4.58GB)
Book Type DVD-RAM
Media Type DVD-RAM
Available Write Descriptor zCLV 3.0x 4155KBps
Data area starting sector 31000h
Data area end sector 22211Fh
Linear Density 0.280um/bit
Track Density 0.615um/track
Number of Layers 1
Complete Media Code
00000220 0C 09 0A 0B 0B 09 0A 0A 0B 4D 61 74 73 75 73 68 .........Matsush
00000230 69 74 61 20 45 6C 65 63 74 72 69 63 20 49 6E 64 ita Electric Ind
00000240 75 73 74 72 69 61 6C 20 43 4F 2E 2C 4C 54 44 2E ustrial CO.,LTD.
00000250 0D 20 20 20 20 20 20 20 20 4D 30 31 4A 33 30 30 . M01J300
00000260 34 0D 20 20 20 20 20 20 20 01 01 82 00 6A 2D 2D 4. ....j--