Which preservative could be added to the excipient based on Guar gum for the preparation of omeprazole suspension?

Guar gum provides a nice stable viscous suspension, but it does require a few days of rest in the refrigerator before the gum is completely swollen. In order to make urgent preparations, it would be useful to create the basis (without Omeprazool). Given the short storage time, no preservative was foreseen in the formula. Also because Aqua Conservans at pH 8.4 would not work Effici ë NT. However, given that the basic suspension is made in advance, a possible preservative might be appropriate. In this way the base can still be kept in stock for a few months....
which preservative can be used, that the acidity of 8.4 is not ï Nvloedt, is stable at this pH é n compatible with Omeprazool?