Although our 17x8.5" ET20 ARC-8 was originally designed for the E30 M3, it's lower offset lends itself well to classic 5s as well. Below are some photos from a test-fitment on one of our E28s.
Test Vehicle: 1984 528e with OEM Brakes & Suspension
OEM Wheel: 16x7.5" ET20 (OEM M5 wheel)
APEX Test Wheel: 17x8.5" ET20 ARC-8 in Hyper Black with 235/45-17 tire
Applies to: All BMW E28 Models
TL;DR: 17x8.5" ET20 with 235/45-17 is our recommended fitment. 3mm or 5mm front spacers required depending on tire brand. Rear fender roll not required & may not rub with certain tire brands, but recommended to ensure no rubbing. Coilovers may require larger front spacers.
E28 Front Fitment
Front inner clearance. OEM wheels (left) vs APEX wheel with 3mm spacer (right).
3mm front spacers are required with this fitment. Without spacers, the tire was lightly brushing the strut and would not be drivable. This tire has a particularly large "rim protector," so it's possible that by using a different brand of tire there may be ample clearance without adding spacers. However we still recommend at least 3mm spacers for peace of mind.
Outer clearance with 3mm spacer (required) installed. No rubbing occurred during our street test drive. Click images to enlarge.
E28 Rear Fitment
Rear fitment looks very promising at stock height. Inner clearance is more than sufficient, so fender clearance is what we'll focus on. No spacer was used for the rear fitment. Click images to enlarge.
With the rear suspension nearly fully-compressed, we see the potential for rubbing. For this test-fitment, there was still a piece of paper's worth of clearance between the tire and fender. No rubbing was felt or heard during a street test drive. Just like with the front, a different brand of tire may offer a few millimeters more or less sidewall. So, it is possible to run this fitment without rolled fenders, but we recommend being prepared to lightly roll the inner lip if any rubbing is experienced.
The 17x8.5" ET20 with 235/45-17 tires with 3mm-5mm front spacers is an excellent square fitment for the E28 chassis. Rolled rear fenders are not necessarily required, but recommended for a trouble-free fitment. Wider tires can be used with larger spacers and rolled fenders. Narrower tires can be used for an easier fit.
At stock ride height compared to OEM M5 wheel mounted on the rear (left). Photoshopped mild drop (right).