Have owned my TR6 for just over two years and in that time have always ran it on Esso E5 with 'Millers Oils' lead additive. The previous owner always put additive in the fuel but it turns out that some time prior to his ownership the head was converted to unleaded. I only discovered this when I went through the extensive file of previous bills and discovered the bill for the conversion.
I try not to run the tank lower than 1/4 full, and never fill it over 3/4 full, but last week I had to put some petrol in (the tank was lower than 1/4) but had no additive to hand so just put a few litres