My JPI shows about 1500-1550 for exhaust temperature on most cylinders, franklin 165, when I slightly lean the fuel. What is the optimum temperature I should try to maintain that is best for the engine longevity?
It's likely that information was never developed when the engine was designed and tested. On low compression, normally aspirated engines, leaning the mixture for max rpm and smooth engine operation is the way to go. Especially if you are using leaded fuel. Using egt to monitor engine performance is a nice feature and can be useful to identify issues with individual cylinders.
EGT will vary from day to day, and even runs. It will be mostly the same, but will depend on air temperature, density altitude, and humidity; not to mention placement of the probles. As Carsten mentions, the best way to lean is to use max (either RPM or peak EGT on a cylinder) and then richen to get a smooth operation.
I don't know which JPI you have, but usually for best economy (usually under 75% HP) it's lean to first peak EGT. I have a single probe EGT, and pretty much the peak EGT matches the lean for max RPM and smooth engine operation.
If you want to really dig into this topic, check out Saavy Aviation. He provides an in-depth look at what the EGT is telling you.
My summary (for whatever that’s worth!), the EGT is very useful for diagnostic information and leaning. The actual “numbers” are not very important at all, in fact the very first EGT gages did not even have numbers, just tick marks.
We have the Electronics International EGT, I just happen to run about the same temps as you. Maybe our mechanics are like minded and placed the probes in exactly the same spot on the exhaust!