What is the term for oil that is too thick for proper engine lubrication?

The term that best describes oil that is too thick for proper engine lubrication is "sludge." Sludge forms when oil breaks down from heat and contaminants, creating a thick, gooey substance that can impede the flow of oil through the engine. This can lead to inadequate lubrication, resulting in increased wear and tear on engine components.

While viscous oil does refer to oil having a greater thickness or resistance to flow, it does not specifically imply that it has degraded into a harmful substance like sludge. Heavy oil could also refer to oils that are thicker but may not necessarily indicate that they are unsuitable for engine use. Contaminated oil includes any oil that has foreign substances in it, but again, does not specifically denote the thickness or improper lubrication characteristic highlighted in the question. Thus, sludge is the most accurate term in this context.