My 1997 Toyota Landcruiser is a 40th edition with an automatic transmission & triple-locked differentials. It has 333K miles on the 1FZ-FE gas engine but has been serviced well (from what I know) through its life. It has always been in the Southeast (Atlanta, Chattanooga,  & now Nashville). The previous owner said that he was told it had a very small rear main seal leak but I have not seen evidence of this. The top of the engine has been rebuilt & head gasket replaced recently. It runs very well but I have yet to drive it more than about 100 miles since I've had it. 

So, my question: would you recommend running ATS CRO 505 through it before changing the oil? I have some Amsoil 5W-30 signature series oil I intend to put in it along with the recommended Amsoil filter (EA15K51). The engine sounds great for its age but the PO told me that he was running 15W-40 through it in the summer so I'm concerned about that thicker oil sealing better than if I clean out all the carbon & junk followed by the thinner (but factory recommend weight) oil. I've run both CRO & CRF through my 2015 Tacoma along with the Amsoil oil & it runs like a champ. 

Thanks for you advice!

Paul