Oil leaking from seals or rings causes preignition too because it becomes a low octane fuel .Because this is happening at low revs when the engine is working harder this is a clear sign. Any good 20W/50 will work fine. Do not bother changing the rails unless the bonded surface has worn through (rare on these engines) . Normally I only change the banana rail on the tensioner because the nylon cover wears out.
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