When engine runs hard it consumes oil.... Over the period of 5000 kms, upto half a liter consumption is considered as normal.... 150k kms is nothing for Honda R18 engines.....
Looks to be "CAT CON " exerting back pressure and hindering acceleration if the spark plugs & air filter are genuine and replaced along with assuming valve clearances done.
10-40W will make the situation more worst by making it more slugish do not go on this route....
" Shifting to thicker out of spec Motta Tail is usually done due to budget constraint of not being able to overhaul the engine" but it detoriates the engine much faster so rather than being a temporary remedy it is unknowingly be harmed by the owner.
Best is to check the compression pressure. If still the engine is found weak, start lube oil replacement at early intervals
