The 9XX 4-cylinder and V8 engines have bearings and lubrication systems that are designed for a high flow rate more than a high pressure, and a heavier viscosity oil. The _W part of the viscosity rating can be as low as you wish, but the numbers to the right of the W should be 50 or higher.
In a later Service Bulletin, Lotus expanded the range of acceptable oils to include 40 wt, but in the same SB, specifically recommended Mobil 1 Motorsport 15W/50, and Castrol Formula RS 10W/60. Neither of those products are still available, and the current Mobil 1 15W50 can't hold a candle to the old "Motorsport" variety. The best Mobil 1 for the engine is Mobil 1 20W-50 V-Twin Motorcycle oil. That's the old Mobil 1 that can no longer be API Certified because it contains more than twice the API's current (green) maximum ZDDP level. The Castrol Formula RS 10W/60 has morphed several times, and the name has changed. It's evolution is still a great oil, but Castrol doesn't sell it in North America. However, BMW rebrands it as it's "Motorsport" oil that is required for it's M-engines. You can buy it at any BMW dealership. It's great stuff, but expensive.
Even if the overnight temperature drops to freezing or a little below, a 20W/50 synthetic is not too heavy, and the heavier 50 or 60 wt is better for the engine once it's up to temperature. If the daytime highs are consistently below freezing, then it would be best to have the _W part lower (0W, 5W, or 10W). But for most northern-tier Lotus owners, by that time, the fun cars are heading for Winter storage.
Regards,
Tim Engel