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The botch +4 plugs that you talked about don't seem to work well for Japanese engines but from what I read do work on some American style engines (valve lay out, combustion chamber shape) the SAE (society of automotive engineers) publishes books and papers, some libraries can get these, they are well worth the read as they are peer reviewed papers based off of real research and testing, they cover alot of questions about exhaust size, spark plug style, combustion chamber shape, tires, fuels, oils, the list just goes on and on! but their info is not free, they sell their research papers, so there isn't alot to find for free on the internet, but it's all solid info.
Basic their only oil viscosity is that you need to keep your oil pressure up to protect your bearings, the thiner the oil is the easier it flows out of the bearing, most cars don't have oil pressure gauges any more and the pressure switch is just an on/off pressure switch.
Air filters are a hot topic of debate as well, but it really comes down to the amount of air going in to the engine has to burn a set amount of fuel, so if the restriction is at the air filter or at the throttle plate it doesn't really matter unless you are running wide open throttle, personally I like the best filter I can afford as I like to keep as much dirt out of my engine as possible, so small holes and alot of area, I used to have a foam air filter but I sold that car and it was the best air filter in my mind.
Most people will say that they have good luck with warm air intakes, they work in theory and in practice, Honda used to have a warm air intake as part of their intake design on all of their cars, it would maintain a 100F air temp and you could tell when it was not working because your mileage would drop like a rock.
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