Good thread!
When I first tried window cleaning in 1996 I used a 1987 Volvo 340 1.7 petrol saloon which was great but which would inexplicably "die" randomly every few days and which I never got sorted. It would always start again after ten minutes or so but it would be awkward if you were in town on a roundabout etc! The tank was drained, the fuel filters cleaned, carb checked, pump checked but it never played up when you were working on it. That lasted two weeks and so:-
First window cleaning car was a 2.3 petrol Volvo 240 Estate "D" reg (1986) when I started in 1996. It had electrically heated seats (well, only the driver's one worked), a manual choke and I managed to eke 28 mpg out of it.
Next was a Honda 2.2 Automatic Accord "J" reg (1991) with aircon which I bought in 2001 and which was the first car I did WFP out of in 2005 with a 175L tank and a couple of 25L barrels. It was rubbish on fuel and 22 mpg was about it.
It got written off by a French lorry driver in 2006 and I put the £500 payout (LOL) down on my (then new) present Fiat Doblo 1.3 diesel (45 mpg) which is on lease 'til October 2010.