Atomic four has no oil filter. Since the oil capacity of the engine is so small I change the 2 3/4 quarts mine holds at every full tank of gas. That works out to about 30 hours and has worked for me 15 years. Forget about adding aftermarket oil filters. The way they are adapted into the oil circulation system on the Atomic Four makes them very inefficient. They filter only the oil that is in the "bypass" circuit not the complete supply of oil going to the oil pump. The small increase in total engine oil capacity (the volume of the filter and lines) they offer is a benefit though. Their added complexity and several lines and fittings are potential failure points that could cause a loss of oil pressure and a big mess in the bilge.
There is nothing wrong with simply changing the oil every 30 hours. I use straight 30 weight for the last change right before winter layup and after the weather warms up in May I switch to straight 40 weight. It is important with any marine engine to change the oil right before layup so the engine hibernates in fresh acid-free oil over the winter.
My engine has always consumed oil, and with no reserve capacity in my tilted Vee Drive configuration, I check the oil every couple of hours running time. I do have the aftermarket PCV system. It keeps the engine compartment and cabin free of any engine smells. A pilot told me a PCV system is not recommended for engines like ours (and airplanes) that run at a constant speed. I often wonder if the PCV system has an effect on my more than average oil consumption.
When I adjusted my valves I noted the interior of the engine was very clean. Many Atomic Fours have trouble with oil gumming up the valves. Mine runs clean so adding a little oil between changes is acceptable to me.
The metal blue suction can works best for me for changing the oil. It is sturdy metal and the oil sucked out through the dipstick tube is well contained for disposal. It is perfect for my lawnmower and snowblower too. Just watch the suction tube! It can ooze oil long after use.
No doubt your Atomic Four has had lapses in maintenance like mine has by previous owners. It is no worry if the engine runs well today. The Atomic Four is a very dependable engine. Like all engines it requires regular use to remain in good condition.
