Here's an article I found on FordMuscle.com:
Holley heads and oil consumption
The Systemax heads we installed back in January (along with an Explorer intake) ran very well out of the box, powering our then AOD '88 LX to a 13.4 @ 102 on puny radials. That was with no other induction changes -stock cam, throttle body and mass air meter. However soon thereafter our motor started consuming oil at the rapid rate. We knew immediately that the problem was with the valve guides in the Systemax heads.
As with Trickflow heads, Holley heads feature valves that are rotated in the chamber to bring them closer to the centerline of the cylinder. (Holley Systemax heads have 17 degree valve angles, stock heads are 20
Poor valve train geometry on the Holley Systemax heads led to the valve guides wearing out in just 10,000 miles. The result was oil consumption at the rate of one quart per 200 miles.
degrees.) The idea behind this is to unshroud the intake valve, and thus increase airflow into the cylinder. While the concept works, the problem is the twisted valve changes the valve train geometry. The result is the rocker arms place an increased side load on the valve guides because of the reduced distance between the centerline of the valve stem and the centerline of the pedestal/stud mounting boss.
We discussed this problem at length with Brian Tooley of Total Engine Airflow (coincidentally he was an engineer at Holley, working on the Systemax project, before he left to start his own company.) Brian told us that the problem is made especially worse when customer mills the Holley head, or installs it on a decked block, or uses non-stock cam and rockers. All of these modifications require properly checking the valve train geometry to select the correct length pushrod. Brian also pointed out that not all rocker arms are equal, even though they may have the same ratio. His comparison of several different brands of 1.6:1 rocker arms showed they all had different overall lengths (between the centerline of the stud and the roller tip.) This does not mean the rocker arm is more or less than 1.6:1, but that the length of the arm changes the sweep of the roller tip across the valve stem as the valve opens and closes. This affects the amount of load places on the valve guide, and thus it is critical to measure using special valve train geometry tools. What is interesting is that Holley does specify a longer than stock pushrod when installing the Systemax heads on a stock 5.0L engine. Unfortunately even this does not truly correct the problem of accelerated guide wear.
Brian indicated that while it is standard practice to check valve train geometry on a highly modified race engine, it is not realistic to expect consumers to check it for what is supposed to be a "bolt on" stock replacement head. The only real fix for this problem is to re-design the head so that the rocker mounting pad is moved in correlation with the revised valve angles. Brian indicated that Trick Flow had the exact same problem when it first released the pedestal mount version of its Twisted Wedge Head. Ultimately TFS discontinued the pedestal mount head and now offers a stud-mount head with much improved geometry.
Needless to say, ten thousand miles after installing the Systemax heads the motor was burning one quart of oil every 200 miles! We had even converted the '88 to a T5 in this time span, in the hopes of knocking on the 12's with our seemingly strong running combination. However the one and only trip to the track with the Holley head and T5 combo resulted in a 15.2 @ 100 - a dramatic example of how much power is lost due to excessive oil consumption.
92 Hatch On3 Turbo, T56
92 Vert 349 Lunati Stroker T5
87 Gt Eddy HCI T5
87 Notch in progress