Dealer recommended oil differential change at the 30,000 mile service before inspecting. After I insisted on inspection, front was “very dirty” and rear not so much but I had them replace both. Did I waste my money?
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NO…………………………
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Given the condition of the front it’s conceivable you were somewhere between right on time and a bit late. Consider checking on shorter intervals.
Manual or CVT? I might wonder about that “vert dirty” comment as the differential oil should not really get that way.
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You believe differential fluid will stay “clear” its whole life . . . ?!
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On my Acura they wanted the rear changed at the first 10,000 miles, then I do the trans, transfer case, and differential every 30,000. No you didn’t waster your money. Replacement is expensive.
The opinions I expressed are simply based on my own experience.
Haivng worked for 3 Subaru dealers I changed and checked differential oil on a very regular basis and never saw “very dirty” diff oil unless there was an internal seal problem or someone botched an engine oil change.
I’ve owned 3 Subarus and never had a dirty diff oil problem even at 100k miles. Nor have I ever had a diff problem even at 240k, 250, and 300k miles with any of the.
One anomaly I did see was brown fluid at about 3k miles on a rebuilt Subaru trans though. This rebuild had been done in CO and the trans locked up when the owner returned to OK. OIl was brown and owner was upset. With trans on bench in pieces things looked very strange and made no sense.
It took me several days of on/off thinking before that Eureka moment hit. I took the final drive dipstick and started looking over some new/used Subarus on the front line. I discovered that whoever in Denver had rebuilt the trans used the wrong final drive stick. No idea what happened in Denver…
The automatic stick was 3/4" longer than the manual. This meant the diff oil was showing FULL but in reality was about .4L down.
I suspect that here the “very dirty” oil was a sales pitch because the front diff is a piece of cake to change.
Will it hurt? Nope; not a bit. Was it needed? Not likely IMO…
The OP insisted on inspection. What were they going to say? The diff fluid is clear and fine? They were already doing the upsell before the car entered the shop.
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Probably an unnecessary expense, but you did want assurance, and fresh fluids of the correct type do that. What does your owners manual call for? If, say, every 60,000 miles, start counting now.
I’d argue that you can never “over change” oils in your car. Meaning, it’s far better to change your various fluids too early/often than not enough, or never.
Of course, since the fact remains that any car could be wrecked tomorrow, I guess you could make the argument that all maintenance is a waste of money. But that’s a pretty cynical way to live life. 
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I drive a 2014 c7 Corvette stingray…about 4k miles a yr…5yrs 20k…dealer wants me to change oil and filter more than once a year…it’s a machine wouldn’t hours run…(miles) be more exact and in line with most other pieces of equipment. Tractors planes etc . Dlr almost pushes me to change as though the use was far more severe then it is…What do you think…2,000 mile oil change or hours run?
You are driving a very expensive vehicle and in your manual it should say xxxx miles or xx months . What more do you need . And why did you tag on to a Subaru thread ?
Fluids age with both miles, time, and faster with short distances between infrequent starts. That’s why short haul driving warrants more frequent heavy duty service intervals - the oil sump collects moisture and raw fuel from blow-by that isn’t burned off and becomes corrosive, and some fluid additives need time at operating temperature to do their job. Metals exposed to engine coolant experience localized corrosion when there are long periods without circulation at operating temperature, etc. Brake fluid draws moisture from the air (hoses and polymers are somewhat moisture permeable and some gets past the seals) and the swept surfaces of brake calipers and cylinders corrode and pit if they sit too long. Even the AC should be operated frequently to avoid shaft seal damage and leaks. 6mo. intervals is not too often for a car that sits around, especially with winters or a moist climate - if you want to know for sure you can have a sample analyzed.
I don’t think you often need to worry about A/C getting run too infrequently considering it turns on automatically when you run the defroster
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Those who store or operate cars infrequently should consider exercising the AC periodically. There also are older cars where the AC comes on only when commanded to (I own one).