This has been talked about before. The X1/9 is a flat tappet engine so modern oils, no matter what brand, will not properly protect this engine that was designed in the 1960's. The consensus from the experts, including Bernice from this forum, is the Shell Rotella 15-40. I have always thought that this was because of the high use of ZDDP, which was gradually reduced in motor oils over the years. One expert's opinion (I don't know his name, just copied his opinion, as well as others into Word files) is that higher ZDDP in the diesel 15W-40 is the reason why the cams last longer. But, people freaked out when the ZDDP was reduced in the Rotella to make it safer for the Cat and O2 sensor. The real difference is the HTHS (High Temperature High Shear). Again quoting an oil expert's opinion, "SAE xx rates oil viscosity at 100C/212F and HTHS rates oil viscosity and shear at 150C/302F." From Lubrizol Additives 360: "High temperature high shear (HTHS) viscosity of engine oils is a critical property that relates to the fuel economy and durability of a running engine. The drivers behind lowering HTHS viscosity are new global governmental regulations to improve fuel economy (FE) and lower greenhouse gases (GHG) in new vehicles. Lower HTHS viscosity tends to improve FE and lower GHG but higher HTHS viscosity affords better wear protection so a careful balance must be found when formulating an engine oil. Sufficient HTHS viscosity is critical in preventing engine wear in the critical ring/liner interface area by maintaining a protective oil film between moving parts. Think of the protective oil film as if you are trying to swim. If the film is too thick like molasses you can barely move and have to expend a lot of energy; too thin and you sink to the bottom. What you want is the right balance of support and ease of movement. The oil has to be thick enough to maintain separation of the critical moving parts but thin enough to allow for fuel efficient operation."
All SAE 0W-30,5W-30,10W-30,0W-40,5W-40 and 10W-40 ratings have the exact same minimum HTHS requirements. (2.9 at 150C/302F). So, a 10W-40 is NOT necessarily any thicker at 150C than a 0W-30. An SAE 0W-30 that is also ACEA A3/B3, A3/B4 or C3 approved has a minimum HTHS requirement of 3.5 at 150C/302F. So an ACEA A3/B4 approved SAE 0W-30 is actually required to be much thicker at 150C/302F and provide far better HTHS shear protection than an SAE 10W-40 that is not ACEA A3/B4 approved is required to provide. YES, a 0W-30 can be thicker than a 10W-40! SAE 15W-40 has special requirements that exceed the individual "15W" and "40". They include a minimum HTHS of 3.7 at 150C/302F. I know this sounds complicated as it did to me. I'm not an oil engineer.
Typical SAE 10W-40 and SAE 20W-50 products are discouraged by virtually every OEM and they are not approved by any OEM for use in modern cars. So, many oil companies that make them use their cheapest base stocks in these products and the result is inferior protection when compared to other modern oil products. These 10W-40 and 20W-50 products do NOT share the ZDDP limits that the more common ILSAC (International Lubricant Standardization and Approval Committee) approved 5W-30 and 5W-20 products have, but the absence of those strict ZDDP limits in 10W-40 and 20W-50 has not shown any real advantage in camshaft protection.
The ACEA A3/B3, A3/B4 or C3 ratings will insure that the oil provides good protection under HTHS conditions. You can find these ratings in the Castrol Edge 0W-30, Mobil 1 0W-40, and other similar products. These products will be thin enough to allow the flow required to flush away abrasives and will still provide the required high temperature high sheer protection. You will NOT find that approval in Mobil 1 5W-30, Mobil 1 10W-30, Mobil 1 0W-30, Castrol Edge 5W-30, or other similar products. These are NOT the correct oil products for flat tappet lifters, such as our X1/9. So, just using ANY Mobil 1, Castrol Edge, Royal Purple, or what ever brand you are trusting, is not the answer. Only certain products within those lines (and others) meet the requirements.
Besides the Rotella, these are some other oils that are recommended for our engines.
AMSOIL SYSTHETIC ZROD 10W30 (ZRT) (ZRP)
LUCAS ROAD AND CLASSSIC HI-PEFORMANCE
ROYAL PURPLE HPS HIGH PERFORMANCE STREET OIL
VALVOLINE VR1 20W50 10W30
I would like to hear other people's opinions.