Hi.
As Richard has said Shop For Parts are good, and very competitive too. (Bloody Selenia Oil is dear anywhere you go, but I wouldn’t use anything else in these engines.
I have my Oil and Oil Filter replaced twice a year along with the Plugs. Some may say it's silly, but I don't care, at 57 years of age I have done this with ALL of my cars. I am a big believer in preventative maintenance.
And when it comes to Air Filter, well Tom from Angel Tuning fitted a nice system for me that collects the cooler air from the outside via the scuttle tray, makes a Huge Difference I can tell you engine performance wise. (Much better for the engine and combustion than gathering hot air from a very hot engine bay.)