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In August 2017 I replaced manual transmission oil in my 2005 Toyota Hilux, which was at 178,280 km back then. The oil was very good synthetic, top-notch.

Two weeks ago, when the truck had done 205,419 km (27,139 km on the oil), I took oil sample from there and sent for analysis which revealed iron (Fe) at 417 ppm and PQ index 143. The message from the lab was:

Iron level abnormal. PQ Index approaching tolerance limit. All other wear levels appear satisfactory for first sample. Silicon level (dirt/sealant material) satisfactory. Water content acceptable. Viscosity within specified operating range. Action: Drain oil from unit if not already done and evaluate wear metal debris. Resample 20000 kms to monitor and establish wear trend. Note: In the absence of OEM specifications, general severity limits have been applied.

So, apparently the gears have been wearing out. I don't know whether it has mostly happened since the 2017 oil change, or the iron simply wasn't properly flushed out back then (changing oil back then I did not use any flush additive; are such additives even used for flushing manual transmissions?)

The oil itself is still good apart from the iron content. Does changing it now make any sense in terms of making the gears live longer?

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The metal in your oil is not there because you didn't use a flush agent, it's there because your transmission is making metal. The question is whether it's just normal wear or whether you've got gears stripping. The lab message was:

Action: Drain oil from unit if not already done and evaluate wear metal debris.

This is good advice which you should follow: drain the oil and drop the pan, cut the filter open (if there is one, manuals mostly don't) and check for big pieces of metal, check the pan scavenger magnet as well. If you have shavings or pieces of metal then you likely have a problem that needs attention, if there's no visible metal that's a good sign, and I would check it again in 20,000km as recommended.

Changing the oil in the transmission fluid may help you get a bit more life out of it, but it's not the main reason for changing it.

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  • Rare but they do exist @Greendrake. I try to make responses general when I can to cover more circumstances, I'll edit for clarity.
    – GdD
    Jul 4, 2019 at 12:42

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