Thus, a multigrade 5W-30 oil is made from a 5W base oil - measured at zero degrees Fahrenheit - but the added long-chain molecules slow its viscosity loss to that of a 30-weight oil when at 212 degrees Fahrenheit. Most wear occurs during cold-start and warmup, when instead of full-film oil-wedge lubrication, mixed lubrication occurs - part oil film, part surface contact. It is normal for oil under pressure from the engine's oil pump to be supplied to the hollow interior of the shafts, emerging through drilled holes to lubricate every camshaft bearing, and every cam lobe and tappet. [Extracted from the article]
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