Hmmm.... good points about heating the fuel.
Propeller has it right tho, there will be an air gap between the oil tank and the fuel tank - they will remain two separate entities. Some thin insulation in the void might do the trick, provided the heat has somewhere to go...
Mounting a standard SRX twinshock oil cooler might be a good idea - I had an oil cooler on my Aussie SRX but UK ones don't seem to have one for some reason???

. The RHS engine cover casting includes mounting points for oil lines, holes just need to be drilled & tapped.
Also I was thinking of mounting the oil tank on some ally plate with fins poking downwards, to aid cooling - the fins would be directly above the carb intakes so should have decent airflow. This may seem a little extreme but I have a benchtop mill so making the plate & fins should not be difficult.
The size of the void in the fuel tank is only 2.3 lt - so not a drastic reduction on the 18lt standard. It would be good to use the 'saddle tank useless fuel capacity' for the oil tank, but this would mean that you could not lift the fuel tank off the oil tank.
Using the SZR660 or SRX mono oil cooler is out of the question for me - no offense intended but it looks like a goiter. "Aard's SRX" is more like it (thanks for the pic!) - but my hump is much smaller and already full with battery/CDI/reg-rect.
More thought needed - beats working! In the meantime, I am off to pick up some 1mm steel sheet to try and weld up a an airtight tank.
B