Castrol EDGE and AMSOIL Signature Series are both full synthetic motor oils. Below I put the two 5W-30s next to each other using only what each company publishes on its own data sheet or product page. Where a company doesn't publish a number, I say so. I don't fill the gap with a guess.
Quick Take
The clearest published difference is the drain interval. AMSOIL rates Signature Series for up to 25,000 miles or one year in normal service (AMSOIL data sheet, AMSOIL oil change interval). Castrol lists up to 20,000 miles or one year, whichever comes first, for EDGE (Castrol EDGE page). Both list API SQ and GM dexos1 Gen 3 for 5W-30. Castrol's U.S. page doesn't list NOACK, TBN or pour point, so I can't compare those. I have also compared AMSOIL with Pennzoil Platinum, Valvoline, and Mobil 1.
Side by Side: 5W-30
| Item | AMSOIL Signature Series 5W-30 (ASL) | Castrol EDGE 5W-30 |
|---|---|---|
| Drain interval | Up to 25,000 mi, 700 hr or 1 yr (normal); up to 15,000 mi (severe) [1] [3] | Up to 20,000 mi or 1 yr, whichever comes first [2] |
| API / ILSAC | API SQ (Resource Conserving), SP; ILSAC GF-7A, GF-6A [1] | Exceeds API SQ/SP/SN PLUS/SN; ILSAC GF-7 [2] |
| GM | dexos1 Gen 3 [1] | dexos1 Gen 3 [2] |
| Ford | WSS-M2C946-A/B1, WSS-M2C961-A1, WSS-M2C971-A1 [1] | WSS-M2C946-B1, WSS-M2C961-A1 [2] |
| Chrysler | MS-6395, MS-13340 [1] | Meets engine test requirements of MS-6395 [2] |
| NOACK volatility | 5.3% typical [1] | Not published on the U.S. page |
| TBN (ASTM D2896) | 10.4 typical [1] | Not published on the U.S. page |
| Pour point | -48°C (-54°F) typical [1] | Not published on the U.S. page |
Drain Intervals
AMSOIL's 25,000-mile rating is for personal vehicles in normal service. Severe service (fleet use, lots of idling, frequent towing, dusty roads) drops it to 15,000 miles. AMSOIL also excludes modified engines, racing and alternative fuels, and says to change at the vehicle maker's interval outside the U.S. and Canada (AMSOIL data sheet, AMSOIL oil change interval). AMSOIL says not to go past 12,000 miles or a year on another brand of filter. Castrol lists up to 20,000 miles or one year, whichever comes first, for EDGE (Castrol EDGE page). For a daily driver still under warranty, the owner's manual interval is the one to follow with either oil.
NOACK and TBN
NOACK (ASTM D5800) measures how much oil boils off at high heat. Lower means less oil lost to evaporation. The ILSAC GF-7 limit is 15%, and AMSOIL lists Signature Series 5W-30 at 5.3% (AMSOIL data sheet). TBN is the oil's reserve for neutralizing acids, and AMSOIL lists 10.4. Castrol doesn't print either number on its U.S. EDGE page. What Castrol does publish are its own test claims: 3X stronger against viscosity breakdown than the leading full synthetic in the Kurt Orbahn test, and 10X better high stress performance in the Sequence IX test against API SQ limits (Castrol EDGE page).
Cold Flow
AMSOIL lists a typical pour point of -48°C (-54°F) for Signature Series 5W-30 (AMSOIL data sheet). Castrol's U.S. EDGE page doesn't list one, so there's nothing to compare. Here in Florida a cold start is rarely the problem. Heat is.
Switching Oils
AMSOIL says Signature Series is compatible with other conventional and synthetic oils. It also says mixing shortens the oil's life and that it won't support an extended drain on mixed oil (AMSOIL data sheet, AMSOIL FAQ). AMSOIL also says mixing voids its limited warranty. If you switch, drain it all and put on a new filter.
Which One Fits You
If you want the longest published interval and a data sheet with the numbers on it, Signature Series is the one I'd point you to. If you change your oil on the factory schedule and want something off the shelf, Castrol EDGE carries the same API SQ and dexos1 Gen 3 listings for 5W-30. Either way, check your owner's manual for the grade and spec your engine calls for.
Frequently Asked Questions
Is AMSOIL better than Castrol EDGE?
In 5W-30, both list API SQ and GM dexos1 Gen 3 on their own product literature. The published differences are the drain interval (AMSOIL rates Signature Series for up to 25,000 miles or one year in normal service, Castrol rates EDGE for up to 20,000 miles or one year, whichever comes first) and how much data each company prints. AMSOIL publishes NOACK volatility and TBN on its data sheet. Castrol's U.S. EDGE page does not.
What is the drain interval for Castrol EDGE vs AMSOIL Signature Series?
Castrol's U.S. product page lists up to 20,000 miles or one year, whichever comes first, for EDGE 5W-30. AMSOIL lists up to 25,000 miles, 700 hours or one year for Signature Series in normal service, and up to 15,000 miles in severe service. If your vehicle is under warranty, follow the owner's manual.
Can I switch from Castrol EDGE to AMSOIL?
Yes. AMSOIL states Signature Series is compatible with other conventional and synthetic motor oils. AMSOIL also states that mixing shortens the oil's life and that it does not support extended drain intervals on mixed oil. AMSOIL also says mixing voids its limited warranty. Drain the old oil completely and put on a new filter.
Does Castrol publish a NOACK number for EDGE?
Not on the U.S. EDGE product page I checked. AMSOIL's data sheet lists a typical NOACK of 5.3% for Signature Series 5W-30.
Sources
- AMSOIL Signature Series 100% Synthetic Motor Oil Data Bulletin (G2880, June 2026). AMSOIL INC.. https://amsoilcontent.com/ams/lit/databulletins/g2880.pdf
- Castrol EDGE: Full synthetic motor oil (U.S. product page, 5W-30 grade). Castrol USA. https://www.castrol.com/en_us/united-states/home/products/our-brands/automotive/castrol-edge-oils/castrol-edge.html
- AMSOIL Oil Change Interval. AMSOIL INC.. https://blog.amsoil.com/amsoil-oil-change-interval/
- AMSOIL Frequently Asked Questions. AMSOIL INC.. https://www.amsoil.com/t/faq/
Sources checked October 16, 2026. Oil companies update formulas and data sheets. Check the current data sheet and your owner's manual before you buy.
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