6.5L GM Diesel Oil Change Guide

6.5L Diesel Oil Change Procedures & Oil Selection Guide

The older an engine gets, the more important it becomes to adhere to the "severe" maintenance intervals. Routinely changing the engine oil at OE specified intervals is imperative to the life of your engine. Since diesel engines suffer from fuel dilution and soot contamination, one could argue that this service procedure is more critical than it is in other engine types.

Applicable Model Years: 1991+ 6.5L GM/Detroit Diesel
Service intervals:
5,000 miles under normal conditions, 2,500 miles under severe conditions
Oil filter p/n:
ACDelco PF1218
Oil pan drain plug gasket: ACDelco 14090908
Oil fill cap: ACDELCO 12C19
Viscosity spec:
Refer to table below
Engine oil capacity: 7 qts w/ filter for 1991 - 1998 model years, 8 qts w/ filter for 1999+ model years

6.5L GM/Detroit Diesel Oil Selection, Specs, & Procedures

The 6.5L GM diesel has a relatively small engine oil capacity at 7 quarts (8 quarts for 1999+) with a filter change. For comparison, a 5.9L Cummins has a 12 quart system, a 6.9L/7.3L IDI has a 10 quart system, and the 6.6L Duramax has a 10 quart system. Even though the 6.5L has an oil cooler, the oil in these engines may have a tendency to degrade quicker and have a relatively low resilience to fuel dilution and other forms of oil contamination. Heat also plays a large role in the degradation of engine oil, which ultimately modifies its properties.

A synthetic oil is your best choice for any engine, and thus remains our recommendation for the naturally aspirated and turbocharged 6.5L diesels. There's no special procedures for converting to a synthetic engine oil and synthetic service fluids are far superior to conventional petroleum based "dino" oils. Albeit slightly more expensive, synthetic oils tend to provide a greater level of protection for your engine. Amsoil is our oil of choice and their diesel motor oil products are available in one of two tiers:

Amsoil heavy duty diesel oil

Amsoil Signature series diesel oil

Heavy Duty Synthetic 15W-40

Signature Series 15W-40

4X the wear protection than required by Detroit Diesel's scuffing test specifications, reduced oil consumption, greater resilience to mechanical, chemical, and thermal breakdown.

6X the wear protection than required by Detroit Diesel's scuffing test specifications, greatly reduced oil consumption, greatest resilience to mechanical, chemical, and thermal breakdown.

6.5L GM Diesel Viscosity Chart


Ambient Temperature Range

SAE 15W-40

> 0° F (preferred)


> 32° F (15W-40 preferred over 30W)

SAE 10W-30

< 32° F (cold weather viscosity)

While a straight 30 weight oil is acceptable in ambient temperatures greater than 32° F, 15W-40 is largely preferred and recommended in place of a SAE 30W. 10W-30 must be used when the engine is operated in ambient temperatures below 0° F and can be used in place of a 15W-40 when the ambient temperature is in the 0 to 32° F range. Engine oil should be changed every 5,000 miles under normal conditions and 2,500 miles under "severe" duty conditions, which includes frequent towing, extended periods of idling, off-road use, etc.

How to Change the Oil in a 6.5L GM Diesel

Changing the oil in a 6.5L diesel is no different than a standard gas or diesel engine - but if you happen to need reassurance, here are the procedures.

Note - the following procedures are based on a turbocharged 6.5L turbodiesel.
Click any thumbnail to view high resolution, fullsize image w/ details (where applicable)

engine oil pan

• Place a suitable container beneath the engine oil pan (minimum 8 quarts capacity) to catch the oil as it drains.

• Remove the oil pan drain plug with a box end wrench (in this case a 9/16" or 14mm wrench, however the actual size of your plug may vary - it may have been replaced, etc) and allow the engine oil to drain completely. The drain plug points to the driver side of the vehicle.

oil draining from oil pan

• While the oil is draining thoroughly clean the drain plug with a lint free rag. Inspect the drain plug gasket and replace as required. The best practice is to replace the drain plug gasket at every oil change. Replace the drain plug and tighten snug once the engine oil has drained completely.

• Reposition the drain pan as necessary and remove the engine oil filter. An oil filter wrench may be required to loosen it. Another method of getting a better grip on the filter is to wrap it with emery cloth or sand paper - especially useful if the filter is slick with oil.

engine oil fill location

• The engine oil fill cap is located at the front of the engine, just to the passenger side of the thermostat housing. Removing or loosening the cap may help the oil drain quicker, but make sure to place a rag over the fill tube if you remove the cap completely.

oil filter location

• Using a lint free rag, clean the oil filter mount where the gasket of the oil filter seals against the oil filter mount. Inspect the mount and make sure that the old oil filter gasket did not stick to the sealing surface; this is very common and will result in a compromised seal when the new oil filter is installed.

filling new oil filter with clean motor oil

• Fill the replacement oil filter with roughly 3/4 of a quart of oil. The filter will hold approximately 1 quart of oil, however if it is filled to the brim you are likely to spill it when installing the filter.

• Thoroughly lubricate the oil filter gasket with clean engine oil.

• Reinstall the oil filter. Hand tighten snuggly; do not tighten with oil filter wrench.

oil dipstick location

• Add 7 quarts for pre-1999 and 8 quarts for 1999+ model year engines (minus the amount added to the oil filter) of new motor oil to the crankcase through the oil fill tube at the front of the engine.

• Verify the engine oil level using the oil dipstick.

• Ensure the oil fill cap is secure and that the engine oil drain plug is not leaking, then start the vehicle and check for leaks at the drain plug and oil filter.

oil filter and Amsoil diesel oil

• You can't go wrong with an OEM ACDelco oil filter. This truck received 7 quarts of Amsoil Heavy Duty 15W-40 motor oil and is ready for severe service.

6.5L GM Diesel Oil Analysis

The GMC featured herein only has 60,000 miles on the odometer; very little for a 1995 model year vehicle. Regardless of age, a periodic oil analysis helps indicate conditions in which more frequent service (shorter oil change intervals) may be necessary. In this case, the engine idles regularly for extended periods of time and is used to tow often, thus we wanted to measure fuel dilution at the 2,500 mile severe oil change interval to feed our curiosity.

If we had found that fuel dilution exceeded acceptable standards, it would be necessary to further reduce the oil change interval and send in an addition oil sample until a suitable interval was identified for the truck's operating conditions. If you're serious about achieving the maximum service life from your 6.5L, consider performing an oil analysis once a year. Amsoil offers this service and a single analysis costs under $35 USD. Their oil sample pump, a one time investment, is under $30 USD. The information obtained from an oil analysis may be priceless.

Oil Analyzers oil sample kit w/ prepaid shipping

Amsoil oil sample pump w/ sample tubing