Fuel only gels as a result of extreme cold. The "gel" is the wax in the fuel gathering together causing larger wax particles that can't pass through the nozzle. Sounds more like there is water in the fuel and that mold has started to grow in it. If he has a clean filter between the burner and the tank, there should be no problems, except changing filters until he uses up the bad fuel. My guess is that the burner needs to be serviced also. Larry's suggestion is also going to help, but if the nozzle and screen are partially clogged, nothing is going to help until he has them replaced. If he has water in the fuel, there are products such as HEET that will disperse the water and allow it to flow through the filter and mix with the oil to be burned.