::standfirst Healthy oil pressure is the best single indicator of a T-Series engine's condition. A rough rule is at least ~10 psi per 1,000 rpm, with strong pressure once warm; low or wildly fluctuating pressure points to wear, usually the bearings, oil pump or relief valve, and shouldn't be ignored, as it precedes expensive damage.
What's normal
The traditional rule of thumb for the XPAG and XPEG is a minimum of about 10 psi per 1,000 rpm on the road, once the engine is warm. A healthy engine will comfortably beat that, pressure climbs strongly as soon as revs rise from idle, sits reassuringly high through the mid-range, and holds steady at cruise.
That rule of thumb is an owners' guide, not a factory figure. At hot idle, pressure naturally falls as the oil thins with heat and the pump slows, and how far it falls is a measure of wear: what matters is that pressure comes up firmly the moment you blip the throttle. The published figures are below.
What the published figures say
The factory and club figures do not quite agree, so both are given here.
- Workshop Manual (TD and TF General Data): oil pressure in normal running 40 to 45 psi; relief valve setting 50 to 70 psi. The 50 to 70 psi often quoted as "normal" is the relief-valve figure, not running pressure. See XPAG and XPEG settings and torque figures.
- MG Car Club Y-Type Register, "The XPAG Oil System" (Neil Cairns): the system pressure relief valve is set to operate between 50 and 70 psi when new, which agrees with the manual. It describes a new engine at normal running temperature as showing at least 50 psi at idle and 50 psi at 30 mph in top gear, and treats the hot 30 mph reading as the test to watch. It says the workshop manual quotes a hot oil pressure of 40 to 55 psi "in some cases", explaining the spread by production tolerances. The by-pass valve inside the pump, which protects the engine if the filter blocks, is set at 7 psi. (source)
Where they differ: the Y-Type Register's 40 to 55 psi hot figure is wider than the 40 to 45 psi this archive reads in the TD and TF manual, and its 50 psi at idle and at 30 mph describes a new engine, above the manual's running figure. A warm engine at 40 to 45 psi is within the manual figure; treat the Y-Type Register's 50 psi as what a fresh engine shows.
Watch how pressure behaves, not just what it reads. A firm rise off idle matters more than the peak number.
What low or fluctuating pressure means
Persistent low pressure, a slow decline over the miles, or a needle that swings around in a way it did not use to, none of these should be ignored. Work through the likely causes in a sensible order rather than jumping to the worst case.
1. Gauge accuracy. Before condemning the engine, confirm the gauge. Fit a known-good mechanical gauge in parallel, a dashboard gauge or its capillary line can lie, and it is cheap to rule out.
2. Oil-pressure relief valve. The system relief valve, a ball held on its seat by a spring, lives in the end casting of the oil pump behind a large brass hexagonal plug, according to the Y-Type Register, and dumps excess oil back to the sump. A worn ball or damaged seat lets pressure leak away permanently. Remove the plug, spring and ball, inspect for pitting on the ball and its seat, and reseat lightly if needed. Renew the spring if it has gone soft. The Y-Type Register warns that an extra washer behind the spring is sometimes added by people selling cars, to mask engine wear.
3. Oil pump. A tired pump, worn gears or excessive end-float, cannot generate the pressure the engine needs, especially at idle. On the XPAG the pump body cap is surface-ground and fitted with no gasket; anyone who reassembles it with a paper gasket or without cleaning the mating faces has just given the pump extra internal clearance, and pressure will suffer accordingly.
4. Top-end bleed. A worn rocker shaft and bushes let oil escape from the top-end feed faster than the pump can supply it. This shows up as low pressure at idle in particular, often with a noisy top end. Rebushing the rocker assembly restores both.
5. Bottom-end wear. Worn main and big-end bearing shells are the classic cause of low hot pressure with a heavy rumble under load. If pressure is poor after the relief valve, pump and top end have been ruled out, the shells are the next stop, and the sooner they are dealt with, the less likely the crank journals themselves are damaged.
The right oil
The XPAG and XPEG are flat-tappet engines. Their camshaft and followers rely on the zinc/phosphorus (ZDDP) additive package in the oil to survive; modern low-SAPS oils formulated for catalyst-equipped engines have that package deliberately reduced and are not appropriate here.
Use a classic-engine oil with a proper ZDDP package, most of the major oil companies and specialist suppliers offer one explicitly aimed at pre-emissions engines. A 20W-50 viscosity suits typical UK/European use; hotter climates or a tired engine may take a thicker grade, colder climates a thinner one. Verify against the workshop manual and your own conditions.
Change the oil and the filter together, at least annually or at the mileage interval, whichever comes first. Old oil that has done a season's worth of short trips is more damaging than fresh oil worked hard.
Protecting the bottom end
Oil pressure protects the crank; you protect oil pressure with a handful of quiet habits.
- Warm the engine gently. Do not lean on the throttle until the oil is up to temperature, cold, thick oil does not reach the top end quickly, and the bearings are the last thing you want to load before pressure is fully established.
- Keep to the service intervals. Annual oil and filter changes are the cheapest engine work you will ever do.
- Watch the gauge. Learn what your car normally reads warm at idle, at 30 mph in top, and at a fast cruise. Any persistent shift from that baseline is worth investigating.
- Treat a persistent drop as a warning. A gauge that has quietly lost 10 psi over a season is telling you something. Diagnose it now, at the cost of a relief-valve strip or an oil-pump inspection, rather than at the cost of a crank regrind later.
Related reading
- Cooling the XPAG and XPEG, heat and oil pressure are two sides of the same story; hot oil is thin oil.
- Ignition and Charging, under-advanced ignition dumps heat into the engine and thins the oil in the process.
FAQ
What oil pressure should an MG XPAG have? The Workshop Manual gives 40 to 45 psi in normal running, with the relief valve set at 50 to 70 psi. The MG Car Club Y-Type Register describes a new engine as showing at least 50 psi at idle and at 30 mph in top when hot, and says the manual quotes 40 to 55 psi hot in some cases. Owners also use a rough rule of at least 10 psi per 1,000 rpm.
Why is my T-Type's oil pressure low? Diagnose in order: confirm the gauge, then the pressure-relief valve, then the oil pump (including whether its cap has been reassembled correctly, with no gasket), then top-end wear at the rocker shaft, and finally worn main and big-end bearing shells.
What oil should I use in an XPAG? A classic-engine oil with a proper ZDDP additive package, 20W-50 suits typical use. Modern low-SAPS oils formulated for emissions-controlled engines lack the zinc/phosphorus these flat-tappet engines need and risk cam and follower wear.
Is a drop in oil pressure at idle normal? Yes, within reason. Pressure naturally falls as the oil warms and thins and the pump slows at idle. What matters is that it comes up firmly the moment revs rise and holds steady at cruise; a slow, persistent decline over months is not normal.
Sources
- Workshop Manual, TD and TF General Data, as cited on XPAG and XPEG settings and torque figures.
- Neil Cairns, "The XPAG Oil System", MG Car Club Y-Type Register, checked 2 November 2026.