I recently bought a Simplicity model D walk behind tractor with a sickle bar and 2hp cast iron Clinton Engine. I was planning on doing an oil change, but have no idea how good previous maintenance was and what oil was used. The current fill is very black and sorta goopy looking. The engine runs beautiful and I have yet to even see a puff of smoke. I've only ran it for about 45 minutes. So detergent or non detergent for this oldie? Thanks