So I always see folk saying that dark oil is used oil that needs changing, or that their oil turns dark in 5000 miles or whatever so justifies being changed. But actually oil that doesn’t go dark soon after being installed just indicates it’s not doing a good job of keeping combustion products in suspension and leaving them stuck to the parts in your engine instead, right? Oil that goes dark quickly is good oil.
Not sure if this applies to gearbox and diff oil however, as there shouldn’t be stuff to hold in suspension in the first place.
Do not know anything near enough to comment on engine oils, but when refurbing my 93 Mk1 5 years back, we dropped the open diff lub out expecting Hiroshima treacle and were some what surprised to find it was still semi translucent at 143,000 KM, and no magnetised sludge on the nut!
But then, it’s not being subjected to combustions and carbons. Never the less I had the new stuff ready so in it went. Right now we are obligated Suzuki annual services for new car warranty rules so it’s a shame to see “fresh” lube dropping out (it only does about 1,200 miles per annum) but we have to remember (?) sump oil is, at least I’m told, as hydroscopic as brake fluids? Could be hogwash that one. Interesting subject …good post in my opinion.
I think engine oil is less hydroscopic than brake fluid, but has the benefit that it gets hot enough to ‘burn’ any water off so long as you do long enough trips
Most cars I’ve had, including this ND, which had an oil change about 800 miles ago the oils start to darken after around 500 miles approx. I’d worry on a petrol engined car if it was really black, very overdue a change.