I have used Penrite Gearbox oil 30 for many years in my 403 with a 405 gearbox and overdrive.
I have to disagree very strongly that a well tuned and maintained 2 litre doesn't need an overdrive. The manufacturer clearly saw a benefit when updating from 403 to 405. I haven't driven an 85c with overdrive but the 100A has enough power to make it worthwhile. There are several 400s that are now sporting the very elegant conversion that Derek Hughes pioneered. It would be good to hear some feedback from anyone using one of these.
It may well be that we drive on different road types, but for UK diving with a fair bit of motorway mileage I feel the upgrade to overdrive was a huge improvement. It is vital to change the differential ratio to 405 spec to get the benefit.
The main benefits are lower revs, improved MPG and quieter cruising at motorway speeds but it also makes the car a bit quicker in the lower gears due to the change in differential ratio. I also recommend the use of the magnetic switch that was used on the 405 to drop overdrive as soon as a change down from fourth is made. The vacuum hold switch used on the MGB is arguably better still but these are hard to find now.
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