Oils & Fluids for the AC Aceca: A Quick Reference
The Aceca takes a handful of different oils and fluids, and getting them right matters, the wrong grade or the wrong type can damage an expensive engine, gearbox or braking system. This is a plain-English reference to what goes where, the choices owners commonly make, and, importantly, where the three variants differ. The exact grade, capacity and figure for your own car always come from the workshop manual and the AC and Bristol Owners' Clubs; this page orients you, it does not replace them.
Confirm against your car
Grades and capacities vary by engine and by year, take them from your workshop manual, not from this overview. The three Aceca variants differ, especially in the engine and gearbox. For the brakes, confirm whether your car runs glycol or silicone fluid before you add anything. And if your engine or gearbox has been rebuilt, follow the builder's recommendation over any general advice. Getting oils and fluids wrong can cause expensive damage.
First, know your variant
The Aceca came three ways, and it changes what some of these systems take: the AC-engined car (AE chassis) with AC's own Light Six and a Moss gearbox; the Aceca-Bristol (BE chassis) with the Bristol six and Bristol's own gearbox; and the Aceca 2.6 (RS chassis) with the Ford Zephyr engine and an AC alloy-cased box. Where a fluid differs between them, it is flagged below.
Engine oil
Most owners now run a classic 20W-50 multigrade in place of the monograde the original handbooks specified, a 20W-50 formulated for classic engines, with the higher anti-wear additive level older engines like. Change it by time as well as mileage: on a car that covers few miles, oil degrades sitting in the sump, so an annual change is sound practice regardless of distance. See the engine oil & filter guide for the procedure.
- AC-engined cars: a classic 20W-50 is the usual choice; the external cartridge oil filter favours a detergent multigrade. Castrol, among others, publishes a 20W-50 recommendation for the AC Aceca.
- Aceca-Bristol: the Bristol six is a different engine, a good classic 20W-50 is the common choice, but confirm the grade against Bristol's own figures or a Bristol specialist.
- Whether you need a high-ZDDP "classic" oil depends on the engine's design, flat-tappet valvegear benefits most; confirm what your engine actually needs rather than assuming more additive is better. See the fuel, oil & additives page.
- Rebuilt or uprated engines: many Bristol units have been rebuilt (fast-road cams, different pistons). If yours has, use the oil the engine builder specifies, their recommendation outranks any general figure here.
Gearbox oil
This is where the variants genuinely differ, so identify your gearbox first.
- AC-engined cars use the Moss gearbox; the handbook traditionally calls for engine oil (a monograde around 30W) in the box rather than a heavy multigrade.
- The Aceca-Bristol uses Bristol's own four-speed gearbox, a different unit; use the grade Bristol specifies, not the AC-engine figure.
- If a Laycock de Normanville overdrive is fitted (as on many Acecas), it shares the gearbox oil and is sensitive to the wrong grade, the wrong oil can stop the overdrive working. Use exactly what is specified. See the gearbox, clutch & overdrive guide.
Differential
The differential takes an EP 90 hypoid gear oil to the manual's specification. A useful point from AC owners' experience: fill it to its stated capacity, around three pints on many cars, rather than to the marks on the filler dipstick, because the dipstick can read over the true capacity for the AC housing and the surplus is simply blown out of the breather. Confirm your car's capacity in the manual, and don't overfill.
Clutch fluid
The hydraulic clutch shares the same fluid family as the brakes, so whatever your brakes run, the clutch runs too. If the car is on glycol, the clutch is glycol; if it has been converted to silicone, the clutch must be silicone. Never mix the two.
Brake fluid
This is the one to get right, and the one to check on your own car before you add anything, because two incompatible families are in use:
- Glycol: DOT 3, DOT 4 and DOT 5.1. Amber-coloured, absorbs moisture over time, and strips paint. DOT 4 is the accepted modern choice, superseding the original Girling specification, and is preferred for its higher boiling point. DOT 5.1 is also glycol and compatible with DOT 4.
- Silicone: DOT 5 (a confusing number). Dyed purple, does not absorb moisture and does not harm paint. Some restored cars that spend time standing are converted to it.
Never mix the two families: adding glycol to a silicone system, or the reverse, causes swollen seals and a spongy pedal, a safety problem. So before topping up, identify which your car runs: look at the colour in the reservoir (amber = glycol, purple = silicone), check the master-cylinder cap for a stamped specification, and check the restoration or recommissioning invoices. Glycol fluid should be renewed every one to two years, as it absorbs moisture and its boiling point falls; cars with front disc brakes run the fluid hotter, so fresh fluid matters all the more. See the brake fluid & bleeding guide for the change procedure, and the complete brake-fluid guide for the types, checks and how to tell which your car runs.
Cooling system
The coolant does two jobs, freeze protection and corrosion protection, and the second matters as much as the first on these aluminium engines.
- Use a corrosion-inhibiting antifreeze suitable for alloy engines, mixed with deionised or distilled water (not tap water, which brings scale) in at least the proportion the coolant maker specifies for the protection you need. The inhibitor depletes over time, so the coolant is renewed periodically even if it still looks sound.
- Some owners prefer a traditional classic-type antifreeze as better suited to older engines; others run a waterless coolant such as Evans, but waterless coolant requires the system to be completely drained of all water-based coolant first, so it is a considered conversion, not a top-up. See the cooling-system guide.
Greasing and the lubrication chart
Beyond the oils and fluids, the Aceca has chassis and steering greasing points that need attention on the schedule in the manual's lubrication chart, using the specified grease. It is easily forgotten on an otherwise well-serviced car. Which grease, and what to use now the originals are obsolete, is covered in the suspension and chassis grease guide.
Quick reference
| System | What it typically takes | Watch out for |
|---|---|---|
| Engine oil | Classic 20W-50 multigrade | Grade varies by engine (AC / Bristol / 2.6); for a rebuilt or uprated engine, follow the builder's recommendation |
| Gearbox | The grade your box specifies, Moss, Bristol and 2.6 boxes differ | A Laycock overdrive shares this oil and is fussy about grade |
| Differential | EP 90 hypoid to spec | Fill to capacity (~3 pints on many cars), not the dipstick; don't overfill |
| Clutch (hydraulic) | The same fluid family as the brakes | Must match the brakes, glycol or silicone, never mixed |
| Brakes | DOT 4 glycol, or DOT 5 silicone if converted | Identify which your car runs before topping up; never mix families; renew glycol every 1–2 years |
| Cooling | Alloy-safe corrosion-inhibiting antifreeze with deionised water | Protects the aluminium engine as well as against freezing; waterless coolant needs a full conversion |
| Greasing points | The grease on the lubrication chart | Don't overlook periodic chassis and steering greasing |
Sources. This reference draws on the AC and Bristol workshop manuals and the experience shared by owners in the AC Owners' Club community.
A general reference to help you order the right oils and fluids, not a substitute for the workshop manual, the AC and Bristol Owners' Clubs, or a specialist. Exact grades and capacities vary by engine and year; the brakes are safety-critical; and if anything is in doubt, confirm it for your specific car before you fill it.
About the author
A lifelong classic-car enthusiast in North Yorkshire, near Thornton-le-Dale, who owns and maintains an AC Aceca, writing the single-model reference he always wished existed. Read the full bio.