carbon deposits.. isn't because of the direct injection design? the 86 / BRZ has both direct and indirect injections to prevent this carbon build up problem.
the subaru turbo engines and many other direct injection turbo engines (Audi / VW) have this carbon deposit problem. Hence why VW / Audi only recommends VW504.00 approved oils (Group 4 base I think and at least $75/5L for Shell Helix Ultra Extra / Penrite Enviro something) to reduce this issue.
I have a catch can to reduce this problem too.
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