Ultra1Plus SAE 75W-90 Synthetic Limited Slip Gear Oil API GL-4, 1 gal.

A: Yes, 75W-90 GL-4 gear oil should generally be suitable for a manual transmission that specifies GL-4 oil, even if it contains additives for limited-slip differentials (LSD). Here's why: GL-4 oils are designed for manual transmissions and transaxles, providing adequate protection for gears and bearings under moderate load conditions. The "75W-90" refers to the viscosity grade, meaning it performs well across a range of temperatures, which is compatible with most manual transmissions calling for GL-4. The inclusion of limited-slip additives—typically friction modifiers—enhances performance in differentials with clutch packs (like LSDs) by reducing chatter and improving smooth engagement. These additives don’t inherently conflict with the GL-4 specification or harm a standard manual transmission. In fact, many gear oils are formulated as dual-purpose (e.g., GL-4 with LSD compatibility) to broaden their usability. However, there are a couple of caveats to consider: Manufacturer Specifications: Some transmissions (especially older ones or those with specific materials like yellow metals—e.g., brass or bronze synchros) may be sensitive to certain additives. While GL-4 oils are generally safe for yellow metals (unlike GL-5, which has higher sulfur-phosphorus content), the LSD additives could, in rare cases, alter friction characteristics and affect synchro performance. Check your transmission’s manual or with the manufacturer if it explicitly warns against friction-modified oils. Performance Needs: If your transmission doesn’t have a limited-slip differential, the LSD additives won’t provide any benefit—they’re just along for the ride. This won’t cause harm, but it’s worth ensuring the base GL-4 properties meet the required standards (e.g., API GL-4 or equivalent). In practice, many modern 75W-90 GL-4 oils with LSD additives are widely used in manual transmissions without issue, as long as they meet the GL-4 spec. If you’re still unsure, you could cross-check the product’s technical data sheet or contact the oil manufacturer to confirm compatibility with your specific transmission. So, barring any explicit manufacturer restriction, it should work fine.