CHT increase
 

Since i got my plane I have put 200 hours on it and use as a guide scale where the needle pointer is on the printed letters "CYLINDER HEAD TEMP" markings on the CHT gauge as a benchmark of operating temps at different phases of flight,I have noticed in cruise this CHT consistently sits in the green range with the arrow between the E and the A of the HEAD part of the markings, however over the last several flights in cruise it is running slightly hotter between the A and D markings (still well withing the green range) concurrent with the slight increase in CHT during cruise ( all settings being equal RPM/Leaning OAT ETC) I have noticed during engine run up the left mag has a slightly higher drop then usual , more than the right but again well withing tolerances. What factors could cause this new slightly higher cruise CHT reading ?

Note the engine is 200 SMOH all other engine/air/electrical/fuel systems were renewed at OH

As you have noticed a change in the Mag drop the first thing to check is the Mag timing, both to the engine and the internal timing.

Eliminate any errors with the mags before proceeding futher.

Also worth a look under the bonnet to see if any of the baffling is lose or cracked.

All else being equal, the CHT and oil temperature will rise and fall with ambient temperature as the rate of heat transfer is proportional to the difference from ambient.

So you would expect a seasonal variation as the ambient temperatures change.

If that doesn't explain it then there could be one or more other explanations e.g. gauge error, baffling leaks, induction air leaks, magneto timing, valve sticking/leaks .....

Are there any other changes such as poor starting or idling?

grass cuttings, sand and dirt covering 3/4 of the air filter. all back to normal. I am amazed how much !!!! it picks up

I initially found your explanation a bit odd, as a restricted air-inlet would richen the mixture.....surplus fuel - cooling......but, of course, if you are running lean of peak and then richen..:8

CHT increase
 

I thought it was odd also as I imagined the restricted air will give a richer mixture which would be cooler but the air filter is the only thing I have changed and its back to running at the lower temp it was before. My leaning method is basically screw it out till it runs rough then back it in a tad to pick back up the higher rpm . So I'm not sure why it is was doing what it was doing but happy it's back to normal

What engine type is it?

G

Lycoming 0-360 C4f