If it is a part with a regular maintenance schedule, it should be designed for maintainability.

Most maintainability conflicts come from packaging and design for assembly.

Efficiency more often comes into conflict with durability, and sometimes safety.

Right but what I'm getting at is that there can be tradeoffs that might make designing for maintainability mean optimizing for something less important to the end user.

Do you optimize an engine for how easy it is to replace a filter once or twice a year (most likely done by someone the average car-owner is already paying to change their oil for them), or do you optimize it for getting better gas mileage over every single mile the car is driven?

We're talking about a hypothetical car and neither of us (I assume) design engines like this, I'm just trying to illustrate a point about tradeoffs existing. To your own point of efficiency being a trade with durability, that's not in a vacuum. If a part is in a different location with a different loading environment, it can be more/less durable (material changes leading to efficiency differences), more/less likely to break (maybe you service the hard-to-service part half as often when it's in a harder to service spot), etc.

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manquer 4 hours ago | parent [-]

Only TCO matters, that is the efficiency you actually optimize for, ie dollar per mile[1]not miles per gallon.

If the car is going to need to be in shop for days needing you to have a replacement rental because the model is difficult to service and the cost of service itself is not cheap , that can easily outweigh any marginal mpg gain .

Similarly because it is expensive and time consuming you may likely skip service schedules , the engine will then have a reduced life, or seizes up on the road and you need an expensive tow and rebuild etc .

You are implicitly assuming none of these will change if the maintenance is more difficult , that is not the case though

This is what OP is implying when he says a part with regular maintenance schedule to be easily accessible.

[1] of which fuel is only one part , substantial yes but not the only one

	▲	its_ethan 4 hours ago \| parent [-]
		I'm just gonna copy and paste a response to another similar comment: The point that I am making (obviously, I think) is that tradeoffs exist, even if you don't think the right decision was made, your full view into the trade space is likely incomplete, or prioritizes something different than the engineers. Putting some random number of hypothetical mpg improvement was clearly a mistake, but I assumed people here would be able to get the point I was trying to make, instead of getting riled up about the relationship (or lack thereof) of oil filters and fuel efficiency.