6.5 yr old VW/Audi battery in my 95.5 S6 gave up the ghost (guess that's not too bad...Battery Tender helped longevity). I've researched Interstate batteries vs. Audi battery (191-915-105Q...old part no.). The battery appears to be a BCI Group 41 (size: 11-3/16"L x 6-7/8"W x 6-7/8"H). Since battery is located in car's interior (under rear seat cushion) it needs to be vented which the VW/Audi OEM battery is (hole near top on short side to accommodate plastic elbow which vent tube attaches to). Battery's height is also critical due to clearance required to underside of seat cushion frame.
Interstate MT-41 will fit but is not vented. Interstate MTP-91 is vented...vent tube connects @ center of battery's long side (rather than short side as on OEM) @ top which also happens to be the location of the battery's vertical hold-down clamp. Interstate does have a vent tube kit which I'm thinking about getting...there may be enough clearance btwn the clamp and where vent tube kit connects to battery for it to fit & then connect to existing flexible plastic vent tube. The MTP-91 is same dimensions except for length (3/8"+/- shorter than MT-41) so wouldn't think that's a problem. MTP-91 is also more powerful and has a longer warranty.
Any thoughts /experiences on Interstate vs. OEM or for that matter other brands that you've found to fit and accommodate venting requirements?