380 SEC maintenance

[freeride]

Before I'll turn my SEC into SDC, I will continue to run it's gas engine for many more miles as I drive a lot. It currently has 200k miles. I would like to take possibly best care of it at this stage.

I know You guys hate oil threads but I bought this car without history, I don't even know what oil is in there now. My question is should I go mineral or synthetic?

It seems like a little risk eitherway, if it has synthetic now then it's bad to go with mineral and opposite... I was thinking Mobil 1 5w-40?

Also, I don't have manual for it, how much oil does it take?

And finally, can You recommend the rest of HQ fluids, I want to do transmission & rear end but don't have numbers.

Thanks