Why do you want this? Do you want to make sure your filter is working? Do you want to monitor the filter to know when it is time to replace it? Do you want to use it in different scenarios such as different speed of gas or transparence of the tubes?
I am not sure if you need a laser - common LED may work as good as laser diode. You may try to use LEDs of different colors and different detectors (photoresistor, another LED, photodiode) possibly with (instrumental) op-amps and see if there is a difference between dirty gas before the filter and cleaned gas after it (or ambient air). But unless you have vast knowledge you cannot be sure what are you in fact measuring - particles of the oil? CO2? Some strange byproducts of burning? O2? A bit better may be comparing gas after new and exhausted filter...