In Reply to: I heard that thick oils like 15W50 is more for.... posted by dks on June 14, 2000 at 00:21:26:
Keep in mind that the engine itself does not know or care whether it's 10 below zero or 125 degrees above zero as long as the oil performs by lubricating and cooling. The reason you must use different oil viscosities IF using petroleum based oil is because petroleum oil will break down much quicker at higher temperarures than will synthetic oil. Everything a petroleum oil will do - a synthetic oil will do much better.
It can safely be determined that BMW has designed the engine to use a 30 weight oil (petroleum in colder weather). Therefore since synthetic oil protects at ALL temperatures (both low and high) there is no need to use a synthetic oil higher than 30 weight.
As for high revving engines, the M3 does have a fairly high revving engine but synthetic oils naturally have much better oil film ratings against breakdown than the petroleum oils. Since a lighter oil flows faster and cools better, using a synthetic oil that lubricates, protects and does everything better than petroleum is going to be the choice for any engine including a high revving engine. And since 30 weight is a BMW acceptable weight oil to be used in the engine then a "synthetic" 30 weight oil is all that is required to fully protect the engine.
To go another step, BMW (as well as other car makers) are factory filling their engines with 30 weight synthetic oil to be used year-round regardless of temperature climates. BMW has contracted with Castrol Oil Company to make their BMW "labeled" 5W-30 synthetic oil that is used for factory-fill and will be used by all BMW retailers for all oil changes in cars that came with synthetic oil from the factory except the (E39) M5. This includes the other high revving "M" powered Z cars.
Using any oil that has a high first number (before the "W") is just asking for damage at the bearings and cam lobes in particular. When an engine is first started, this is when the most damage occurs. A dry (cold) crank-up needs oil to flow as quickly as possible to lubricate these critical areas. This is just as true even if it's 125 degrees in Palm Springs or 10 below in Syracuse!
A 15W oil will not flow near as quickly as 0W or 5W when the engine is not warmed-up even on a hot summer day. Don't be concerned about the first number being too low. Most people misunderstand this. This is only the weight of the oil when it is cool like in an engine that has set overnight (even in the summer's warmth). The upper number is the weight of the oil at normal operating temperature of about 100 C (212 F). A 0W-30, 5W-30 or 10W-30 oil will all be a 30 weight oil when at this operating temperature.
There is no need for a thick, high weight oil such as 15W-50 in the M3's engine if using a synthetic oil.
: high revving engines. But if you don't high rev that often, just use a less viscous oils like 10W30 or 5W30 all year round. And thick oils protect your engine better, but takes a longer time to warm up at car startup.
: I might be wrong though, anyone else got an opinion?