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In a 20W50 viscosity, the 20W indicates the oil's viscosity when cold (0 F), and the 50 is it's viscosity when hot (100 F). When selecting an oil for your 907, it's important to keep the second number at 50 or above, and 20W-50 is the default spec (BMW's house brand 10W60 oil for their 'M' series engines is excellent, but expensive).
If you live in a cold climate (real arctic cold, not southern chill), and actually drive the car in cold weather, then dropping the 20W part of the viscosity to 10W or 5W will help the engine start more easily on a cold morning. But once the engine is up to temp, it's the 50 part that counts, and the __W part is no longer relevant.
There are different approaches to bearing and lubrication design. Most modern engines that must meet mandated CAFE standards (Corporate Average Fuel Economy) are using lighter and lighter oils, which then require tighter and tighter bearing clearances and higher oil pressure. It's a 'complete', inter-related design package. Using a thicker viscosity oil in those engines does not work.
All Lotus engines prior to the recent Toyota powered models (all 9XX 4-cyl, Lotus-Ford Twin Cam, Coventry-Climax, etc) use bearing/ lubrication designs that are at the opposite end of the spectrum. The oil of choice has a relatively high viscosity at normal operating temperatures. That requires more open bearing clearances along with a lower oil pressure, but with a high flow rate.
Many owners of vintage cars & older Lotus seem to think that since their modern car is using a 0W20 or lighter oil and the engine lasts a long time, that oil must also be better for their (insert vintage Marque) engine. NO, that's not true! The new Honda is designed and manufactured for water-thin oil, and the 907 was not. Don't try to force either one to be what it's not.
Each engine is a complete design package, each needs what it needs, and the 907 needs a 50+ viscosity oil at normal operating temperature (ie, when hot). 10W40, 10W30, 5W30, and lighter modern oils are NOT better for it regardless of what an overzealous oil company tech rep will tell you, will lead to excessively low oil pressure, and possibly rod-knock. Do not go there.
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The 907 is also a flat-tappet cam design that uses pretty ordinary metallurgy. It depends upon extreme pressure / anti-wear additives in the oil to keep wear in check. Keep the phosphorous level up (the P in ZDDP). The Z and the P are not present in equal balance, and an oil with 1300 ppm of ZDDP will have about 1200 ppm of phosphorus... which is the minimum you should run in your 907. The Valvoline VR1 oil mentioned earlier is in that ZDDP range.
The old Mobil 1 20W50 that Lotus recommended contains more ZDDP than the API now allows for automotive use (more than VR1), so it has been re-branded as Mobil 1 20W50 V-Twin Motorcycle Oil. It's still the same great stuff, you just need to shop for it in the motorcycle aisle.
Most modern oils are lower in ZDDP than what should be used in the 907. Even oils that claim to be 'Added ZDDP' are only 'added' relative to the super low level to which they cut it back a few years ago. They took a lot away, put a little back in, and claim 'Added ZDDP'. BS, don't fall for it.
You want 1200 ppm phosphorus (1300 ppm ZDDP) minimum in your 907 (ie, the current API SN maximum) or more. More can be had via certain select motorcycle oils, or certain racing oils.
Some oil companies blend oils with more ZDDP than the API allows, then market it as non-certified racing oil 'for off road use only' (WWNNSNM - Monty Python). I call them boutique racing oils. They still have all the anti-sludge, detergent and other additives a long change interval street oil requires, but with a high level of ZDDP. They're safe for extended drain intervals in street use. Valvoline VR1 (NOT VR1 NSL), Mobil 1 Racing Oil, Redline Racing Oil, Castrol Racing Oil, and others are examples of those boutique racing oils.
A real racing oil is an 'event' oil. It only stays in the engine for one race event, and is changed afterward. It doesn't need the street additive package, so it doesn't have it. If you use a 'real' racing oil, then change it every 500 miles or 3 months, which ever comes first.
At levels above 1500 ppm, phosphorus becomes corrosive to the metals used in engines. Long term, it can be bad, but the parts don't immediately dissolve. Wear can be bad as well, and the high wear rate that can occur with insufficient ZDDP is worse than the glacial rate of corrosive attack due to too much. You might be wise to save the really high ZDDP racing oils (2400 ppm) for track days, but there's no reason to shy away from a 1750 ppm Mobil 1 for general street use. It has a history of everyday use for decades.
A few current motor oils with high ZDDP levels:
.Phos. / Zinc
2400p / 2500z Redline Racing Motor Oils (Typical, “all have a min. of 2200 ppm Phos” - contains full street additive package)
1750p / 1850z Mobil 1 0W-50 Racing Oil (contains full street additive package).
1600p / 1700z Mobil 1 20W-50 -- old, discontinued,
..................... now Mobil 1 20W-50 V-Twin Motorcycle Oil
1600p / 1700z Classic Car Motor Oil -- http://classiccarmotoroil.com/
1200p / ......... API SF (1980-88) permitted the highest ZDDP levels ever API-specifed,
1500p / ......... with phosphorus ranging from 0.12% to 0.15% (1200-1500ppm).
1400p / 1500z Brad Penn®, Penn Grade 1 Hi-Perf mineral oils
1300p / 1400z Valvoline VR1 Racing Oil ('boutique racing', has street additives, made in full synthetic & mineral oil versions... read the label), API SF/CD.
1300p / 1400z Valvoline VR1 NSL "Not Street Legal" Racing Oil, NO street additives, change every 3 months/ 500 miles.
1200p / 1300z Mobil 1 15W-50 (vintage flat-tappet engines/ Lotus approved).
1200p / 1300z API SL/ SM/ SN “allows” but does not mandate 1200p for SAE 40 and heavier oils. Lighter viscosity grades are restricted to less ZDDP.
Last edited on 03-31-2015 02:21 am by Esprit2