CaptainB said:

The Haynes manual recommends Texaco S5 75W/90 for the IR

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Any synthetic 75W/90 will be fine, and is good for 75K miles. A mineral based 75W/90 will also work, but will need changing every 25K miles or so.

Alex1000 said:

Is there any other way to test the ird if your buying a fl1 with the props removed.

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The only quick check, is to see how much play there is in the rear output flange. If it's got more than about 6mm free lash, then the IRD bearings are on the way out.

Alex1000 said:

Also if it drives el mono with no noises or bangs from the diff, is the diff okay, or cant you tell untill vcu and shafts are refitted.

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Normally the diff survives ok. But if it if damaged, they are cheap to buy and relatively easy to change.