Hello everyone.

My l405 4.4 v8 this weekend gone few times in restricted performance mode, after restarting the engine fault clears and drives good again until heavy accelaration and pops the fault again.
Not smoking, no weird noises and etc.

Have read the faults and had two codes which are:
P0299 - Turbo/Supercharger underboost.
P006A - MAP mass or volume air flow correlation.

Under visual inspection I have noticed oil in the intercooler pipes and in intake manifold sitting by egr pipe, which made me think that turbo oil seals are failing.
I have unplugged intercoolers by turbo and let it run freely, the right turbo is spinning as fine on idle, sucking and blowing air with no issues, and havent noticed any oil dripping even under higher rpm.
However the left side turbo on idling, doing nothing at all, not sucking or throwing any air which made me think that left side turbo is collapsed and needs to be replaced? But after reading few posts, some says that primary turbo is on the right, and secondary is on the left and works only on heavy load? how true is that? That would deny fact that my left turbo is gone? But still wont explain oil in boost pipes. I took all boost pipes off from turbos, revved it and no oil has came out on any of them.

Took the MAP sensor out, its was clogged with soot from EGR and cleaned.

My main questions are:
1. Is it true that both turbos dont work in same pattern? More like biturbo?
2. How horrible job is to replace them? I am mechanic myself just never done RR before. Engine out job?

Please note, I know that some oil is allowed in boost pipes, but it my mechanic opinion it was too much.

thanks