My M240 sensor got some pretty severe dust/spot issues. Probably changing lenses outdoors in the Atlanta pollen season was the primary cause. Anyway, the air blower system did not help at all with the stubborn, large spots I had.
The solution seemed to be that the sensor needed a more thorough cleaning. I really did not want to send the camera away some place to get it professionally done. Mostly I hate to send my expensive camera away for fear of losing it or getting it damaged in transit. The shipping insurance both ways is pretty significant, and then one must add the fee for the cleaning.
After doing a good bit of research and asking questions in forums, I decided to try the DIY method. Based on what I learned, I ordered a "Full Frame Cleaning Swab kit" from Amazon as well as a gummy stick made by "Eyelead".
The kit got to me in one day, so today I got brave and proceeded to try to clean the sensor.
I cleaned the sensor as much as possible with the air blower first and then used a swab and used it as per instructions. It removed the larger spot and most of the others, but not completely. I then tried a new swab with 2 drops of cleaner as per instructions after vigorously blowing the sensor with the syringe. Better but still not completely clean. Some of the little dust/spots seemed to move around but stayed on the sensor. Again I used the air blower and tried yet another swab. Much better results, but there are still a few stubborn dust specs that won't go away. When the gummy stick by Eyelead arrives, I'll try again and see what happens. Anyway, I'm pleased with the results and I don't think I did any damage to the sensor. Here are some "before" and "after" images of the sensor as well as the cleaning kit.