I too have a number of tapes that squeak when played in a 1530 Datassette, with some so bad that the play rate slows and sometimes stops. I don't have that many tapes but around 10% exhibit this kind of behaviour to varying degrees (none will load). I did try fast-forwarding and rewinding the tapes multiple times with absolutely no improvement. What I found to be interesting is that the tapes do NOT squeak, or appear to slow down, when played in a normal music cassette player.
I read on some forums members stating many reasons for this problem, including cassette cases that have warped, the clear nylon sleeves inside the cassette case being worn or damaged and that the tape itself was physically damaged. If any of these were the reason with my cassettes then I would imagine that the problem would also be apparent with a music cassette player - which it isn't.
Regardless, I went ahead and moved the reels of tape from one of my problematic C64 game cassettes into a donor audio cassette shell (with the audio cassettes' nylon sleeve). I then tried to load the game and had exactly the same squeaking and slowing down that the tape exhibited when in its original cassette shell. So I can only conclude that the problem is the tape itself.
My subsequent research led me to the following YouTube video:
The presenter claims that all tapes were/are produced with an embedded lubricant and that this can fade/evaporate over time leading to significant friction between the tape and the tape pad/head. He presents a method using a specific 3M wet silicone spray to "re-lubricate" the tape. I have not tried this myself yet so cannot attest to how effective or safe this is or if it does any long term damage to the tapes. Just letting you know for your info. Look in the video description as there are plenty of references that appear to back up his assertion that tapes sometimes do need, and can be, re-lubricated.
Also, Last Factory (
https://thelastfactory.com/tape-care-preservation/) claims that "Moisture is the top cause of tape degradation. Moisture absorbed by the polyurethane binder, a process called hydrolysis, degrades the binder of the tape. This may be seen as stickiness, flaking, or heard as
tape squeal." and "As tape ages, in addition to absorbing moisture, it loses internal lubricants. This increases stick-slip and can result in tape squeal." Apparently this is called Soft Binder Syndrome (SBS) and includes Sticky Shed Syndrome (SSS) due to the aforementioned hydrolysis and Loss of Lubricant (LoL). Last Factory sells their own, very expensive, tape preservation fluid that also lubricates the tape.
Interestingly the metal plate (behind the tape pad) in my most problematic cassette is quite rusty. So this cassette has been in a very humid environment and so may well have absorbed more moisture than most. Apparently placing cassettes with SBS into a sealed container with silica gel to absorb moisture may help with the problem.
