I have a 10 year old Coleman Powermate generator with a 10 hp Tecumseh 4 cycle engine. Up until now it has only been used in warm weather and the manufacturer specified 30 weight oil has worked fine. I now want to be able to use the generator in winter weather but I cannot locate the owners manual and don't know what the proper viscosity oil would be. Any suggestions as to viscosity and conventional vs synthetic would be greatly appreciated.