Couple of things to keep in mind about temp guages. In general they are "wet bulb" units which means the sensor bulb has to be immersed in the fluid to give a reading. While the mechanical type of guage is generally more accurate than the electric, they are more complicated to install because of the relative rigidity of the capillary line as opposed to electrical wire. The capillary line should be free of kinks and very sharp bends. If they're there it wouldn't hurt to either straighten them out or, if the opportunity presents itself, replace it. Will a kinked tube read? In all probability yes but kinks are best avoided unless the tube is over the age of consent and no force is used.
The plus side for electrical guages is that in general wire is easier to work with than the capillary tube and, depending on the guage, they can be calibrated by a guage shop. There have been a couple of threads mentioning a gauge shop that might be worth a call to see if they can help with the accuracy.
Depending on how deep your concern goes any guage should be checked for accuracy. As Wilf mentioned the higher the quality (read costlier) the guage is the more likely it is to be accurate.
Joe - Canton makes an
oil cooler thermostat that goes in the line between the cooler and the filter/adapter but it's sort of expensive but looks like a high quality piece.
Just my $0.02,
DonC