I had similar problems with my Plextor 8432T - it couldn't read some old pressed CDs and no CD-RWS.
Because it has no greese I decided to do this..
I took wet a tissue and squeezed out some water so that it doesn't drip and gently wiped the lens in a circlular motion a few times, and finally took a dry tissue and did the same. The lens looked much cleaner.
I also used the same method on the edges of the spindle hub (rubber grips), but had to manually rotate the spindle as I applied the tissue to get them nice and clean.
Now drive reads all discs properly and even the CD-RWs.
I took pictures of the cleaned state:
Anyway I should also mentioned that it isn't always to do with dust / dirt. I have a Plextor 755A which recently couldn't read some old pressed game CDs, but noticed the LiteOn drive I have reads all of them perfectly. I investigated further and my findings were:
- all old (around 1995) but pristined CDs except for one which has a small hole on the label side and after installation the game could not run,
- using my CD tool to view sectors I noticed the subchannels displayed garbage further into the CDs (probably where there ar slight damage),
- reading speed was very slow in imgburn, Nero and PlexTools with max speed setting. Good parts read at 8x and problematic areas read at 1x,
- quality scan with PlexTools on Plextor 755A gave CUs on all CDs,
- quality scan with KProbe on LiteOn gave no CUs on all CDs,
Because my Plextor 755A is out of warranty, I applied the cleaning method I described but it didn't change a thing. I decided to adjust the CD laser pot. Problem was there were 2 of them; one for DVD and the other for CD. I measured the ohms (resistance) using a multimeter and noted down the reading. Using a small precision flat blade screwdriver I turned both pots a tiny amount (like 1mm) clockwise (towards the legs I measured). Sure enough the new multimeter readings were a lower value (about 50 ohms lower).
After putting back drive, wow all CDs can be ripped at max speed and also the PlexTools quality scan gave no CUs.
Sorry I forgot to take photos - maybe next time.