I'm currently near the end of a shoot with 2x Sony F65s and we have experienced this exact problem. We have been using oil-based SPFX atmosphere in our sets heavily.
We've sent the cameras back to the local Sony Service Centre twice to have the OLPF cleaned in each camera. The blue-ish tinted filter in front of the mechanical rotary shutter is not the OLPF. It is actually a Band Pass Filter. Cleaning that did not solve our blooming issue. The OLPF actually sits between the mechanical rotary shutter and the sensor. As a result, you need to take apart the entire optical head block of the camera to service/clean it. This is absolutely not advisable to do in the field without proper technicians. The cameras need to be sent back to Sony and cleaned in a dust-free clean room.
I've been talking to a senior service engineer for the HD and Digital Cinematography Camera Systems Department at Sony in New Jersey. He apparently has cleaned lots of F65s that come out of rental houses in LA, especially after they have been used in dusty and sandy areas like New Mexico. And he says he finds mostly dust on the OLPF.
Based on my experience so far, it appears the sensor area is not entirely sealed to the exterior environment. There are two air filters below the lens mount (see attached picture) where the air intake is. Changing them out might help.
But my opinion is that if those air filters can't even filter out fine dust/sand particles, there is no hope of it filtering out even finer contaminants like oil-based SPFX atmospheric haze.
As far as I understand it, Sony has significantly reduced the amount of support they provide for this camera. Finding parts (such as the air filters mentioned above) is difficult. I have a colleague that finished a shoot recently with this camera and had to bring along a technician to clean the sensors every night. They even tried to get support from Sony Japan with limited success.
I feel cinematographers and camera assistants need to know this is a problem for this camera. Even after our second sensor clean, the problem started re-occurring after 3 shoot days.