I'd suggest depends on how hard it the disco is driven, i.e lots of offroad, heavy sand, towing heavy caravans etc. I'm no expert but if you answer yes to any of those then I would suggest doing regular megaflush.
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I think its reasonable to recommend 50K fluid changes on all the driveline.
If you want to take it from a mechanics view point and personal preference, I do all my vehicles, my Pa's Defender, my mum's Jeep all at that 50K for driveline and 10K or 6 monthly for engine.
but at the end of the day, I dont force or use any gloom and doom tactics on customers, i throw it out there as an option.
Some of this post is also in the Sticky: Disco 4 Automatic Transmission FAQ - 8 Speed:
I am about to do a second partial flush of my 8 speed at 100k km. I have been provided with the following fluid:
"Auto Fluid, FEBI, ZF 8, 9 Speed Synthetic Fluid, Bilstein includes ATF supplement added to oil"
Has anyone anything to offer on this alternate to Lifeguard 8?
Am I being anal about not wanting to use it when it comes a well respected independent?
I have some Lifeguard 8 left over from my first partial flush & stood it next to the FEBI for comparison. The colour is similar but the smell is different.
Attachment 152454
Also been supplied some new transfer case oil & have stood the old bottle from the previous change next to the new bottle. Would it be normal for such a variation in colour as shown below?
Attachment 152455
The older oil was used in Post #1 in June 2018. It shows the same colour as the photo taken then.