[paulgato]

1. A mechanic once wrecked my manual transmission by filling it with ATF instead of gear oil after a clutch change job. (The original gear oil was coloured red so he thought it was ATF and didn't bother looking it up in the book.) I realised the error and got it swapped out for gear oil after 100 miles or so, but the damage was done. Gearbox was always noisy after that. It worked OK, but it whined badly. Gearboxes are not all the same, and you experiment with wrong lubricants at your peril. The original Minis had a combined gearbox and engine and they both therefore ran on standard engine oil, which got changed out at every service.

2. I have a 2002 diesel VW Golf Estate, manual transmission. VW say the gear oil never needs changing. After 150,000 miles and at ten years old, I decided to change it anyway. I used the (synthetic) stuff VW recommend and sell for that gearbox, plus I added a small tube of Molyslip gearbox additive (watch out - I believe the 'Molyslip' available in North America is different from that available in the UK, where I am). The difference in rolling resistance was immediately obvious. A very cheap and quick 'mod' and very worthwhile. Conclusion: this story VW tell about transmission oil never needing to be changed is utter *****, probably designed more to sell new cars than to help owners of old ones.

3. You were talking about fitting some kind of temperature-sensitive switch-over device, but you already have one of those. It's called a coolant thermostat. Why not simply run the gearbox heater in line with (or in parallel with) the radiator hose, and block the airflow to the radiator? Always better to prioritise engine heating until engine design temperature is reached.

4. How about helping things along by not only insulating the engine but also insulating the clutch and gearbox? That might help even with the normal heating (by conduction and friction) but if you are running coolant to the gearbox then there is no risk of ever overheating a fully-insulated transmission as the coolant will cool it as well as heat it.

5. Earlier this year I fitted a 240v coolant heater and pump, which I switch on from inside the house with a little radio control keyfob unit an hour or so before driving off. I thought about adding a small 240v self-adhesive transmission heater pad but never did. (Maybe I could use a coolant-powered one instead. Hmmm - built-in thermostat - I like it. I can just link it into the existing hose run from the pre-heater.)

My 2kW mains engine pre-heater gets the engine up to about 70 degrees C or so. (Normal running temp is 92 degrees.) The preheater thermostat actually switches the heater off (but pump still runs) at 85 degrees and back on at 75 degrees, but once the engine is started the temp immediately drops down to about 70 degrees. Still, that's a very useful degree of pre-heating and mpg is much better over the first few miles than it used to be. However, even with a thoroughly pre-warmed engine, and even in summer, if I go on a longer journey the mpg creeps up and continues to increase for about two hours, and then plateaus. My guess is that everything is warming up - engine, gearbox, driveshafts, CV joints, wheel bearings, tyres - and it takes two full hours to achieve maximum temperature, and therefore maximum mpg.

6. My hunch is that over a 7-mile commuting journey you will have very little spare heat to play with. You can barely heat up an engine to normal operating temp in that time, let alone a gearbox as well. I'm running a diesel, and diesels are much less inefficient at lower engine temps than petrol engines (as far as I understand at least.) If you can't pre-heat the engine, then the priority has surely to be getting that engine up to a good temperature as quickly as possible. I like the idea of THEN using excess engine heat to warm the transmission, but I can't see that happening in 7 miles. Maybe block the radiator grille aggressively, and fit an under-engine fairing if there isn't one already, thereby raising the air temperature in the whole engine bay, ...maybe that would help?