Sending 80F water back to a cast iron boiler is severe abuse, and you can ruin it in one heating season or less(!). With 130F return temps a gas fired boiler is going to be OK, but with oil it could be an issue.
I'm going to guess that rather than plumbed primary/secondary with a separate pump driving the boiler loop, when only the radiant is running you have one pump and a mixing valve. If that's the case the flow on the boiler loop will be ridiculously low, and it'll overshoot the high-limit aquastat's setpoint as soon as the pump stops (or even before.) With separate primary-loop pump to guarantee flow you'd be able to mix the return water to temp with the boiler bypass, and the boiler would cycle on/off during extended calls for heat from the radiant, but you would be able to tweak a minimum return water temp under all operating modes.
See if you can sketch up and post a legible system diagram, showing every pump, manifold, and valve (including check-valves, even if they are incorporated into a pump, as is sometime the case.)
BTW: I don't quite understand the rationale for moving from a cheaper cleaner-burning fuel to a more expensive dirtier fuel. Even 5 years ago the cost of heating with gas was substantially below that of oil. Although the Chicken Little/Henny-Penny analysts predicting $200/bbl oil when it crossed $140/bbl during the summer of 2008 should have been ignored, those who were calling the subsequent crash to $28/bbl "a return to normal" were also not credible sources, in view of the steady march of world demand exceeding growth that could ever be profitable at price points that low. (Even at $50/bbl the oil sands of Alberta would have to wait.) Neither $4 heating oil nor buck-a-therm gas will be around forever, but even $2/therm gas in an 80% burner is substantially cheaper heat than $3 oil in an 85% burner. The trend line on both is up, not down, but the back-story on regional gas supplies s much rosier than any argument you could make about heating oil. In most cases converting to gas pays for the conversion in under 2 heating seasons.