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Re: 10W40 or 15W50 is a better choice in summer
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Posted by Sjer [Email] (more from Sjer) on Wed, 9 May 2001 16:52:09 Share Post by Email
In Reply to: Re: oil changed from 10W30 to 15W50, Dick in Falls Church, Wed, 9 May 2001 10:43:50
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Dick, the analogy of sipping through a straw is not a good one. The Saab oil pump is a positive displacement pump. That means it will deliver whatever pressure is necessary to pump the oil (hence the need for a relief valve to prevent catastrophic results in the event of blockage). You must use the appropriate viscosity oil, you will get screwed at either extreme. When oil is too thick (cold start in morning), the backpressure caused by high pressure drop (pressure drop increases with increasing viscosity) through the oil distribution lines will be high and this will limit the oil flow to hydraulically limited parts (there will be plenty of total flow, just not the desired balance to each part). Top end noise at start-up has been well documented on this BB, and as expected usually goes away after the oil has warmed up. This problem will be greatly compounded if there is any plugging in the oil distribution lines. When oil is too thin (0W30 in the summer comes to mind), the pump output is limited by slip around the pump rotors. In this case the pressure drop is relatively low (low viscosity and low oil flowrate) so the low flow users like the top head get enough flow, but the total flowrate is too low and inadequate lubrication and cooling may result. The Saab oil pump was designed for optimal performance at a particular viscosity, and that is reflected by the recommended weights. The recommendations naturally depend on the ambient temperature because the oil viscosity varies with temperature. USE THE RECOMMENDED WEIGHTS!
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