had to remove valve cover for the first time to replace gasket and valve seals of my 2004 Mercury Grand Marquis.
Now it's 363k miles, using 5w20 Motorcraft syn blend and recent 3 years 10w30 to reduce oil loss.
you put 300Kmiles in a mercury. That says the oil did a good job even before you pulled off the heads. Fact that it is not sludged up or thick coated varnish is a testament to the oils of today. nothing up top makes me think the engine has any oil related problems anywhere else.
Wonder why the OP is using 5w-20 in Dubai. You'd think 5w-30 or 10w-30 would be better in that heat unless he's just using up some 5w-20.
I believe the OP meant he used 5w20 during most of the life of the engine but in the last three years switched to 10w30. The ambient temperature in Dubai gets to 106 F according to Weather-spark, but the everyday temperature inside the engine is 210 F. Who-da-thunk you can get that many miles on mostly 5w20.
My main question is-are they the original timing chains & guides? Also, why did the cam need to come out? Valve seal replacement?
No need to remove cams for VS seals., I did it on my Tbird (pretty much exact engine under the intake manifold). There is a tool to tilt the valves.
That mileage isn't spectacular. I know some cabs upstate here pushing 500K. Original everything. They are 98-02 (you can tell by the dished wheels). The transmissions have the drain pan with the plug.