Looks like I have a leak on my valve cover gaskets on the rear corners. Sadly the tensioners were just down about 2000mi ago so I am hoping they didnt over torque and crack the plastic in some way.

Anywho! I am asking about what brands are the most recommended for gaskets. I want to make sure I dont have to do it again.

Anyone have experience with the Mahle kit from FCP? FCP Mahle Valve Cover Kit

 

I've had good luck with parts from FCP but I think I used a fel-pro set when I did them on my 1999 XJR.
I'll see if I can find the reciept just to check.
For some reason my set came with an orange and a grey cover seal and I can't honestly remember if the ones I took off were those same colors.
It looks as if you have a 2001 so you might need different parts than I sourced anyway.
If the covers are intact I would imagine any seal would work, over time though I'm sure some brands would harden and start to weep.
One thing I do remember from the time I did this was finding a lot of small cracks in both covers while I was cleaning them up.
You might have hit the nail on the head with your guess about the cover(s)...

 

My advice is to use bolt washer seals from a 4.2 but, **DO NOT** double up our stock 4.0 bolt washer seals to get thicker results. If you can get bolt washer seals from a 2010+ 3.0 or 5.0 engine (any model Jag of those years and newer) those might work out ok. The idea is that the thicker seals will apply more downward pressure on the valve covers and prevent them from leaking as easily.

 

My solution was just to use double gaskets. I had a solid lifter 454 that needed the
lifters adjusted very often, I learned that trick was about the only thing that worked.

 

We don't even know if the cam covers use the early GREY/ORANGE or later BLACK ones.
The cam covers depend on MAKE, MODEL, YEAR.(engine number actually)

 

An older style flat (cork or cardboard) valve cover gasket (like my Ford 302 used) could be doubled up, with some permatex gasket cement to keep things lined up over time but these rubber gaskets fit into grooves in the covers and with their profiles I'm not sure how you'd keep them from rolling.
You might be able to make up your own cork gasket where the stack might look like this; cover, stock gasket, custom cork gasket, upper head surface and get the same effect though.
I've had the new gaskets on for 3 years now (probably only 20k miles) and I could be wrong but observing them getting brittle over time I think it's the time, not the mileage that does them in.
I'd check the covers for cracks, especially near the bolt holes, clean everything thoroughly, try out the boost addict's (above) solution and use some good quality "stock" style gaskets, realizing that's only my opinion of course!

 

I'd suspect the oil used can affect them too.

I say this as it's become a known thing on a Suzuki forum I'm on that some bikes run on fully sync, not the factory specced semi synth have suffered valve stem seals going hard & leaking. It's never happened to a bike run on semi synth that we know of. It might be an additive in the semi keeps them supple? I don't know, just commenting on how similar seals & gaskets have been seen to fail in other applications.