Well, this is a waaay late reply, but I thought I'd share the interesting set up I found to work very well in in an upper-midwestern winter. I had a 69 Bus with a 1600 engine and a stock carb with an external oil cooler coming off of the stock oil cooler journals. I moved to Chicago for several years with the Bustle. I had run the hoses from the old oil cooler mount up and out of the left side vent and mounted the oil cooler out there, below the vent. The Bus NEVER overheated, ever, in the summer and I drove it in California, down to Florida and up through New England. More relevant to the question, however, was how I stumbled on the fact that if I just waited until the first solid week of freezing weather, then just bypassed the oil cooler by running the pipe back into itself, it likewise never overheated in the winter but gave me good warm air from heater. I guess it was just enough rubber hose to run through to cool off in the cold air. I did have an oil temp gauge on it, however. I doubt I wouldhave gambled without being able to know if I was about to roast the engine. About the time the weather changed enough so my cheeks didn;t hurt outside anymore, I re-hooked the oil cooler and all was ready for warmer temps. Yeah, lucky, I know.
Keep yer hands on the road and yer eyes upon the wheel.