The two products measure the oil temperature at different locations, so it's not terribly surprising if you see a relatively small difference in readings. And of course nothing put out by H-D is necessarily very accurate. Case in point, check out the variability of the other gauges on the bikes like fuel, volt, air temp, etc. When they spend about $5.00 on the full set of gauges, you can't expect too much.

The sending unit for the dash mounted gauge is installed where the oil is being returned to the engine after running through the baffles that force it into contact with the metal surfaces that draw some of the heat out of the oil. The dipstick on the other hand tends to read oil that has just been returned to the tank from the engine. Would that account for a 30°F difference in readings? Maybe, maybe not. But in the overall scheme of things, it really doesn't matter. What you should be monitoring isn't an absolute number, but any significant changes from what you normally see. If your gauge usually reads between 210 and 240 for instance, and suddenly you start seeing readings of 280 to 320 on that same gauge, it's an alert for you to investigate to see if something has changed. Oil temperature on an air cooled Harley will never be as consistent as it is on many other kinds of vehicles, unless you install some sophisticated external coolers with thermostatically controlled fans. As long as you use a good synthetic, I don't see any need to get overly anal about oil temps.
btw, it's not terribly difficult to check the relative readings between the dash mounted gauge and the dipstick. Get a pan of boiling water and dip both the sending unit for the dash gauge and the dipstick into the same water bath. That will tell you exactly how much of any difference is due to the different gauges.
Jerry