Lotus issued a TSB back around 2000, updating their oil recommendations for the 9XX 4-cylinder and V8 engines.
"Lotus 900 series 4 cylinder engines and V8:
Choose a multigrade oil with viscosity in the following range:
Cold viscosity; 0 - 15W
Hot viscosity; 40 - 60"
"Lotus specifically recommends Mobil 1 Motorsport 15W/50, and Castrol Formula RS 10W/60."
"In all cases, the oil quality grades which should be met are either European standard ACEA A3, American standard API SL/CF, SJ/CF, or international standard ILSAC GF-3. Check that this is printed on the oil container."
*~*~* End Quote
Note that the oil grades Lotus recommended, specifically API SJ, have higher ZDDP levels than current, modern oils. All Lotus engines V8 and earlier have flat tappet cams with less than modern metalurgy, heat treating or coatings. They depend upon higher ZDDP levels. The current Mobil 1 15W50 is minimally adequate with 1200ppm, but their 5W50 is not adequate with only 1000ppm.
At that time they tested a number of oils, and approved only two
Mobil 1 Motorsport 15W/50
Castrol Formula RS 10W/60
... neither of which is still in production.
The Mobil 1 product has no modern counterpart... it's just not available any longer. However, the old Mobil 1 20W50 Lotus originally recommended has 1600 ppm ZDDP, and is still available as Mobil 1 20W50 V-Twin Motorcycle Oil. It's ZDDP level exceeds the API's upper limit, so the API won't certify it for use in cars... hence the re-brand to V-Twin Motorcycle Oil. But it's still the same good stuff, you just have to shop the motorcycle aisle. Mobil 1 0W50 Racing Oil has even more ZDDP at 1750ppm, and is one of the few modern cross-over "racing oils" that also contains a full compliment of the street additives required for extended oil change intervals.
Those are the only modern Mobil 1 oils you should put in your 9XX 4-cyl or V8 engine. All the others have 800 to 1000ppm ZDDP, and that's too low for the Lotus engines, especially if you drive the car with some vigor.
You want ............. 1200ppm or better.
... don't use any of the following in a non-Toyota Lotus:
Mobil 1 0W-40 ... 1000ppm
Mobil 1 5W-20 ..... 800ppm "Extended Performance"
Mobil 1 5W-30 ..... 800ppm "Extended Performance"
Mobil 1 10W-30 ... 800ppm "Extended Performance"
Mobil 1 0W-20 ..... 650ppm "Advanced Fuel Economy"
Mobil 1 0W-30 ..... 650ppm "Advanced Fuel Economy"
Mobil 1 5W-30 ... 1000ppm "Mobil 1 High Mileage"
Mobil 1 10W-30...1000ppm "Mobil 1 High Mileage"
Mobil 1 10W-40...1000ppm "Mobil 1 High Mileage"
Mobil 1 5W-40 ..... 800ppm "ESP Formula M"
Mobil 1 5W-50 ... 1000ppm
Mobil 1 10W-30 ... 800ppm
Mobil 1 0W-30 .... 1750ppm "Racing " (good ZDDP / Too Lite)
... the following are good for your non-Toyota Lotus.
Mobil 1 15W-50... 1200ppm
Mobil 1 20W-50... 1600ppm "V-Twin M-cycle Oil", SJ
Mobil 1 0W-50 .... 1750ppm "Racing "
The Castrol product Lotus approved is gone/ retired, but has been rebranded a couple of times through the years. The same basic oil is still around as Castrol TWS Motorsport 10W-60 Synthetic Engine Oil. Castrol doesn't market that product in North America; however it is rebranded as BMW's house-brand for it's M-cars (M3, etc), and is available as BMW TWS Motorsport 10W-60 Synthetic Motor Oil through BMW dealerships. Some BMW dealerships stock the house-brand, others stock the Castrol brand. Either way, it's the same stuff... and expensive.
Castrol GT Racing is one of those cross-over "racing oils" that also contains a full street additive package for extended change intervals. But since Castrol doesn't disclose ZDDP levels, I don't know how it compares to Mobil 1's similar racing oil with street additives. If you want a high ZDDP oil, it's probably the best Castrol product available in the USA, but I can't back that up with data.
Amsoil's recommended products for flat-tappet engines that require higher ZDDP levels:
ARO 20W-*50 Synthetic Premium Protection 1266ppm
TRO 20W-*50 Synthetic Racing Oil 1235ppm
AHR SAE 60 Synthetic Racing Oil 1265ppm
Thin viscosity / high fuel economy oils are only good for your engine if it was designed to operate with them. The Lotus 9XX 4-cyl and V8 engines were not so designed. With their broad clearance bearings and the 4-cyl's oil pump design, it's important that you keep the viscosity number to the right of the W at 50 or 60 in all seasons.
50 degree days may be uncomfortable for some owners in southern climates, but they're not "cold", and 15W__ or 20W__ oils are fine. If you're going to drive the car in weather where the outside temps are consistently below freezing (32F / 0C), then resort to a __W40 motor oil. However, don't get on the car hard, as if it's summer.
Lotus Owners Oftha North (LOON)