I’m not trying to make a case against changing any lubricant, but people need to have a clear understanding that no one can judge the quality of a lubricant by color alone. Color MAY be an indicator, but by itself, it’s a poor one at best.
For any doubters, take a look at diesel engine oil 100 miles after a oil/filter change. You’ll see what I mean.
However, I’m a big advocate of preventive maintenance… if in doubt, and the change is affordable and inexpensive… change it.
I agree. Color is not a good indicator. Time. Oil breaks down and suspends tiny metal shavings. I want that out by 30k miles or 2 years at most.
Color is a good indicator of condition but not the only one... Comparing diesel oil that turns black from soot is not the same comparison of gear oil in a sealed differential... that's an Apples to rocks comparison at the very best. Diesels produce soot WAY more than gas vehicles hence why esp in pre 2008 diesels the next day your honey engine oil color is black as tar its from carbon build up Diesels historically have MUCH higher compression than gas engines by nature of how they work. No spark so the air/fuel mixture is COMPRESSED to be used not sparked. When a honey color gear oil gets black its from heat and pressure... Same thing with most ATFs they are red and when you see its completely void of red or much darker its a good sign the fluid is old or worn. You cannot compare the two for anything really.
Again no one said color is the only way... Color, smell, feel, texture, and of course laboratory sampling are needed. PS the dipstick oil looks black on diesels but 99% of the time when I change them on my diesel, tractors, or big rigs the oil still has a close to honey color. The carbon likes to sit near the surface.