1. I have to agree with Judson that changing the oil a couple of times will do as much flushing as you need - unless the engine has been really mis-handled.  Pull a Cam Cover or look at the inside of the oil filler cap and if there is sludge and heavy deposits all over the inside, you really have to worry.  If it is reasonably clean, no worries.
2. Oil viscosity.  Modern engines are designed for very low viscosity oils.  Don't confuse any comments or experience made to modern engines about viscosity to your JH.  Use the recommended viscosity of 20W/50 or 10W/40.  The latter for colder climates such as Norway.
3. Frequency of oil changes is related to more than one issue.  Oil will break down chemically due to several reasons, and will be contaminated with acid, dirt, water and the additives will deteriorate.  Oil is not that expensive, so change it once a year or every 3000 - 4000 miles:
• Mileage
• Ambient temperature
• Environment - dust/moisture
• Time
• Frequent start-ups with insufficient time at full engine temperature
• Probably other issues.......
• When YELODOG is on the track, I change oil every other event - 300 - 400 miles.
4. More importantly, you should worry about the kind of oil used, and what kind of additives are included in the formula.  Critical to the JH engine is ZDDP, which should be about 0.13 %- 0.18%.  Modern automobile engines differ from our old Healeys in two important ways. First, they have to interact with catalytic converters, and second, they have fewer points where metal actually rubs against metal anymore.

The major change is that an additive named zinc-dialkyl-dithiophosphate, ZDDP for short, has been reduced significantly in oils specifically intended for new cars. ZDDP, along with some other metal additives like manganese, mix with the exhaust and gradually clog up the catalytic converter


Oils intended for diesel engines (which don’t have catalytic converters, yet and require ZDDP to minimize gudgeon-pin wear) in the same weights, such as Castrol Syntec Blend Truck 15W-40, Mobil Delvac, or Shell Rotella are recommended for the JH.  If you like synthetic oil, in February 2008, Castrol released a new formula Syntec in 20W50 which is labeled "For Classic Cars" and contains the necessary amount of ZDDP.

If you can't find any of the recommended oils, phone your oil company and talk to an engineer or their sales dept.