I have been burning about 10-15% waste motor oil in my furnace for over ten years now. I haven't had to do any maintenance for several years either. I have a .75 GPH nozzle in it.
When I first started I was a little anxious so I got a spare filter and such but there hasn't been a lot of crud collected. After the second year I took out the large inline filter expecting it to be sludged up but it was still fine and flowed great. Keep in mind that oil drained from cars is already filtered, the dirt should mostly remain in the car's oil filter.
There are three filters in most oil burner installations. The canister type at the tank or the burner, a monel screen in the fuel pump, and finally the sintered bronze or extra fine monel screen in the burner nozzle itself. I have found the monel screens in the oil pump to be usually almost clogged by gummy crud in all the burners I check, and all but my own haven't had (I presume) old oil run through them. An ultrasonic cleaner is best to clean them.
Gear oil is not really that much thicker than motor oil, the measuring scale is different. Thus an SAE 90 gear oil is similar in viscosity to a 40 weight motor oil. However I would choose the thinner oil I could if I had a choice which to put in the furnace. Automatic Transmission Fluid is very thin, as is 0W30 motor oil.
I don't preheat my fuel oil or do any other special treatment. That's why I keep the waste oil down to 15%. One year I used about 25%, but to compensate (help thin it down) I filled the tank with No. 1 fuel oil. The main concern is to keep the viscosity within the acceptable limits of the oil burner. Old oil burner books are great for this sort of information. For instance they explain how, oddly enough, a thicker fuel oil will increase the amount of oil going through the nozzle.
Finally, when using fuel like this it is important to have (or add) a return line from the burner to the tank, so the fuel gets thoroughly mixed as the furnace runs. The waste oils won't settle once they're mixed, but they might not ever mix properly if they never get stirred up in the first place. But that's not a concern with a little kerosene addition.