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A diesel engine can manage a much higher operating temperature, but it is not because of the oil viscosity. The problem that gasoline engines prior to HCCI have is pre-ignition/detonation. To raise the temp of a gasoline engine's coolant to 230 to get a sufficient heat transfer and, therefore, significant amount of water vapor from the boiling, you would encounter significant combustion chamber temperatures. This can be overcome by a number of methods: direct injection, use of a lower boiling point fluid for your energy transfer (steam engine), and giving up on the idea of a steam engine.
As for the oil temp, you could add an additional cooler, but if my memory serves me at all, oil temp is higher than coolant temp, a potential area to harness energy from.
I would also avoid exhaust heat exchangers, as I foresee some issues in power loss do to the exhaust cooling too soon and the gases not acting as intended, and also you would have to locate it post-catalyst. Good luck with the challenges.
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