the OP is getting mucho answers that are all over the place with this topic. But that's ok. Everyone has their own way and their own reasons why they do things a certain way. In a certain way, I don't heat up the gear in hopes of preventing or reducing PIP. I'm not convinced that will really prevent it. I only warm up the glass vial to make it easier to inject with a 25g pin because 25g allows a quicker healing time in the injection sites than 23g does, and 23g definitely causes more bleeding and I don't need my wife finding blood stains on my underwear and then asking me what I'm doing.
In a certain way, I really like the heating pad idea, but it would take too long. I just turn on the hot water, clean the vial stopper while I'm waiting for the water to get hot, and then I hold the vial under the faucet while allowing only the bottom half of the vial to get wet. That way the rubber stopper never gets touched by water, and it only requires that I hold the vial under the running water for 15-20 seconds to thin out the gear a little bit. Another method I've used is to use a bowl that's shallow enough where the vial is taller than the bowl depth is, and run hot water into the bowl, and while it's overflowing into the sink, I place the glass vial ni the bowl. Again, it only takes about 20 seconds for the oil to warm up enough to make it glide through the 25g pins.
I dunno about the microwave idea. Since microwaves heat things from the inside out, I would think that the gear would be compromised using that method. But who knows.